Part of the file name template is eaten up by Python's `%` formatting,
resulting in log files missing the `s` of "stderr" and "stdout".

In order to support --check we need to ensure that registered
vars exist before checking their stdout. Otherwise ansible
errors out.

Make it easier to use a local pypi mirror in the roles listed above

Also renaming the deploy_* plays to the play name

services if it changes
deploy tasks no longer stop the service and will notify the restart
if any task changes

interface

This is so that we can have multiple supervisors running
in different virtual envs

* Instead of using pypi.edx.org why not cache on localhost?
  Seems to work well and it does result in a speed improvement
  for when we need to download packages. This adds the devpi
  role as dependency for edxapp
* For edxapp we md5sum all fo the requirements files and only
  run the pip installs, bundle, npm tasks if any one of those
  files have changed (it treats them as a group)
* Adds convenience symlinks for pip

prefix

Sets up a pypy mirror caching server