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

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