This extends ansible-pull so that it can support using other
source_control modules for checking out a playbook repository
(issue #3372).  This will check to see if the module exists before
it attempts to do the checkout and will exit if the module is not found.
It requires that the module used to check out the repository support the
parameters 'name' and 'version'.  The option -C, --checkout is now
optional and defaults to the module's default behavior for selecting a
branch, tag, or commit value.  For git, this continues to be HEAD.

Other changes include:
* Remove git from help and use generic term(s) where needed.
* Use SortedOptParser from ansible.utils
* More abstraction of common options used between ansible and
  ansible-playbook.