privilege escalation refactoring

Hi,
I just pushed out https://github.com/ansible/ansible/pull/10411, which
does a major refactor of sudo/su and adds pbrun (powerbroker) and
pfexec to the mix.

I hope to not break existing playbooks and at the same time make it
much easier to add new systems to become other users. The new system
uses 'become' as a keyword where before you used sudo or su.
become_method being the one deciding which privilege escalation to use
(default is sudo).

kick the tires, I expect plenty of things to break or not work as
expected, part of this change is simplifying the very convoluted code
we had and normalizing options as in many cases we had 2 and 3
implementations of the same thing. I specially expect issues with
method exe and flags as they did not seem to be implemented for
anything other than sudo.