'group_names' now available as a variable in templates

On the devel branch, you can now do clever things in templates like this thanks to a patch from Jeroen Hoekx:

{% if 'oracle' in group_names %}
This is an oracle node
{% else %}
This is not oracle
{% endif %}

This may be easier for some things than the "first_available_file" trick emailed earlier, but obviously this requires putting something into a group.
It is explicit, not discovered from looking at the host.

This will also might be useful in the "only_if" directive to have a playbook that was MOSTLY the same on all nodes, but had a step
that should be skipped only on certain types of nodes.

only_if: "'oracle' in $group_names"

Etc.

--Michael