skip "module" not found as plugin, id est directory

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.

Provide hints to playbook callers that a playbook execution had
unreachable vs failures. 2 == failures, 3 == no failures, but
unreachable hosts. 0 continues to be all good.

With the command line option "-c local", ansible and ansible-playbook
should never ask for a SSH password even if this is set in the config.

Fixes #3720

we chdir into this path and read the playbook/inventory if a
non-absolute path is given on the command line, that will fail

* Moved the --list-hosts option that is common to both `ansible` and
  `ansible-playbook` into utils/__init__.py (corrects a FIXME)
* Wrote new help text for the --list-hosts option that makes sense
  for both of the commands that it applies to
* Changed the usage argument in `ansible-playbook` so that it is
  setup in the base_parser method the same way that it is in
  the `ansible` executable
* Updated the help text for several options to correct typos,
  clarify meaning, improve readability, or fix grammatical errors.
  In the case of `ansible-pull`, I changed the help text so that
  it adheres to the same standards as the other executables.

Add option to specify inventory.  No default is defined since
ansible-playbook already does this and it allows an ansible.cfg in the
git repository to take precedence.
Overall, this should help ansible-pull work with less setup in advance,
which should be helpful in kickstart scenarios.  Much of this was
discussed in issue #2464.

Allow the group_vars and host_vars directories to be stored alongside the playbook as well as inventory.

A recent change to ansible-playbook to support json extra-vars
also inadvertently broke the script when no extra-vars are
provided. Simply checking to make sure it is defined and truthy
should take care of the issue.

If --extra-vars starts with either a '{' or a '[', it will be assumed
we are dealing with JSON and parse the data as such.

And also be liberal in what ansible-doc accepts as an empty
requirements specification.

While here fix a typo in template module documentation
wrt jinja2 overrides example.

tweak position & look of Requirements in rst
tweak APT's notes as per anhj's wish

make bin/ansible error output consistent for shell module as for the command module when no arguments

Decided to not deprecate legacy template syntax for now.  Backwards compat is important, though the docs will suggest the new way.

Add code to flag legacy templating like $foo.{bar} as deprecated in favor of Jinja2 {{ foo.bar }} so we
can remove the legacy system at a later date.

Change the retry logic to write a limit file, able to source limit from files by indicating with @filename

When playbooks fail, attempt to create an inventory file in the inventory directory that allows rerunning
of the playbook against only the hosts that failed.

of "-" to be less visually heavy (unless cowsay is installed of course)

one by one - asking for confirmation along the way.

also allows to 'continue' after a certain point

return DOC and EXAMPLES as a list
add moduledev explanation
more