This patch contains actions suggested in review comments
  at ansible#763
  plus some more

In display_callback/module.py, avoid modifying os.environ

Remove parent class AWXDefaultCallbackModule, because it makes
  less since when there is not a job vs. adhoc class split

As a consequence, only 1 parent callback class is needed
  the class which is the default, in absense of awx_display

Move modifications of self.env from
  the runner module to the config module
  to be most consistent with the rest of the code base

Hook in stdout callback volume mount

Share pdd and plugin mounts with the NONE base execution mode

Remove PYTHONPATH assertions no longer relevant

Attempt to patch up docs
Refinement of docs page for the refactored display callback

This changes the location of the mount for the stdout callback
prior value was at
/home/runner/.ansible/plugins

new mount location is at
/home/runner/.ansible/plugins/callback

This is more targeted and more desirable for this reason.
We cannot mount single-files like awx_display.py,
due to permissions error, which
  may be because changes are needed in the Dockerfile
  or some other reason

To carry this out, new utils method callback_mount is introduced
  to make such changes easier to carry out in the future