Some environment changes to speed up deploys

* 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
updating from master, resolving conflicts
skip some tasks when the edx-platfor checkout hasn't changed
Turns the supervisor role into a parameterized role

This is so that we can have multiple supervisors running
in different virtual envs
removing pip cache dir
fixing doc string
with_items typo
Merge pull request #390 from edx/jarv/edxapp-env

Some environment changes to speed up deploys