Validate setting is removed from a configuration

I'm working with some Dell OS 9 switches. I'm attempting to remove a command from the configuration. That part works. The part I'm having problems with is validating it was indeed removed from the configuration.

How do I do a validation check to ensure and command does not exist or is not contained in a result.

For example, I am removing the line from the configuration: "neighbor MUX_IPV4Clst[1-20] distribute-list external-prefix out", but there are multiple group names that will match and all are being removed.

- name: show running bgp
  dellos9_command:
    provider: "{{ cli }}"
    commands:
      - "do show run bgp | grep MUX_IPV4Clst3 | grep distribute"
# wait_for:
# - result.stdout ne 'distribute-list'
  register: output

- name: debug
  debug: var=output

- name: check bgp
  shell: "echo {{ output.stdout_lines[0] }}"
# when:
# - output != "distribute-list"
  when: output.stdout.find('distribute-list') != -1

TASK [removeprefix : show running bgp] *****************************************
task path: /mnt/c/Users/emarq/Documents/Source/Repos/Solutions.Network.Automation/MAS/Ansible/dell/force10/roles/removeprefix/tasks/main.yml:27
Using module file /usr/lib/python2.7/dist-packages/ansible/modules/core/network/dellos9/dellos9_command.py
<10.10.234.98> ESTABLISH LOCAL CONNECTION FOR USER: emarq
<10.10.234.98> EXEC /bin/sh -c '( umask 77 && mkdir -p "` echo $HOME/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531 `" && echo ansible-tmp-1479914628.77-259380380959531="` echo $HOME/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531 `" ) && sleep 0'
<10.10.234.98> PUT /tmp/tmpgRL22M TO /home/emarq/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531/dellos9_command.py
<10.10.234.98> EXEC /bin/sh -c 'chmod u+x /home/emarq/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531/ /home/emarq/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531/dellos9_command.py && sleep 0'
<10.10.234.98> EXEC /bin/sh -c '/usr/bin/python /home/emarq/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531/dellos9_command.py; rm -rf "/home/emarq/.ansible/tmp/ansible-tmp-1479914628.77-259380380959531/" > /dev/null 2>&1 && sleep 0'
ok: [rr1-n22-r13-4048hl-5-1b] => {
    "changed": false,
    "invocation": {
        "module_args": {
            "auth_pass": null,
            "authorize": false,
            "commands": [
                "do show run bgp | grep MUX_IPV4Clst3 | grep distribute"
            ],
            "host": "10.10.234.98",
            "interval": 1,
            "password": null,
            "port": null,
            "provider": {
                "host": "10.10.234.98",
                "ssh_keyfile": "/srv/tftpboot/masd-rsa.pub",
                "transport": "cli",
                "username": "admin"
            },
            "retries": 10,
            "ssh_keyfile": "/srv/tftpboot/masd-rsa.pub",
            "timeout": 10,
            "transport": "cli",
            "username": "admin",
            "wait_for": null
        },
        "module_name": "dellos9_command"
    },
    "stdout": [
        "do show run bgp | grep MUX_IPV4Clst3 | grep distribu\u001b[52D \u001b[55D$run bgp | grep MUX_IPV4Clst3 | grep distribute\n neighbor MUX_IPV4Clst3 distribute-list external-prefix out"
    ],
    "stdout_lines": [
        [
            "do show run bgp | grep MUX_IPV4Clst3 | grep distribu\u001b[52D \u001b[55D$run bgp | grep MUX_IPV4Clst3 | grep distribute",
            " neighbor MUX_IPV4Clst3 distribute-list external-prefix out"
        ]
    ],
    "warnings":
}

TASK [removeprefix : debug] ****************************************************
task path: /mnt/c/Users/emarq/Documents/Source/Repos/Solutions.Network.Automation/MAS/Ansible/dell/force10/roles/removeprefix/tasks/main.yml:36
ok: [rr1-n22-r13-4048hl-5-1b] => {
    "output": {
        "changed": false,
        "stdout": [
            "do show run bgp | grep MUX_IPV4Clst3 | grep distribu\u001b[52D \u001b[55D$run bgp | grep MUX_IPV4Clst3 | grep distribute\n neighbor MUX_IPV4Clst3 distribute-list external-prefix out"
        ],
        "stdout_lines": [
            [
                "do show run bgp | grep MUX_IPV4Clst3 | grep distribu\u001b[52D \u001b[55D$run bgp | grep MUX_IPV4Clst3 | grep distribute",
                " neighbor MUX_IPV4Clst3 distribute-list external-prefix out"
            ]
        ],
        "warnings":
    }
}

TASK [removeprefix : check bgp] ************************************************
task path: /mnt/c/Users/emarq/Documents/Source/Repos/Solutions.Network.Automation/MAS/Ansible/dell/force10/roles/removeprefix/tasks/main.yml:39
fatal: [rr1-n22-r13-4048hl-5-1b]: FAILED! => {
    "failed": true,
    "msg": "The conditional check 'output.stdout.find('distribute-list') != -1' failed. The error was: error while evaluating conditional (output.stdout.find('distribute-list') != -1): 'list object' has no attribute 'find'\n\nThe error appears to have been in '/mnt/c/Users/emarq/Documents/Source/Repos/Solutions.Network.Automation/MAS/Ansible/dell/force10/roles/removeprefix/tasks/main.yml': line 39, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: check bgp\n ^ here\n"
}
        to retry, use: --limit @/mnt/c/Users/emarq/Documents/Source/Repos/Solutions.Network.Automation/MAS/Ansible/dell/force10/removetorprefix.retry

PLAY RECAP *********************************************************************
rr1-n22-r13-4048hl-5-1b : ok=9 changed=5 unreachable=0 failed=1