Update git source repos to https

* Now that edx-platform is fully open source, the right thing to do is
  have read-only http URLs be the default and get rid of all the
  nonsense ssh-oriented infrastructure for config/deployment.
* This patch makes all the git repository references use the read-only
  https urls and removes the git_ssh and 'deployment' ssh key kludges.
* Also contains updates to edx-west/edxapp_prod.yml which make its
  behavior less mysterious. (but this isn't the important part)
Merge pull request #118 from edx/fixing_git_repo

Update git source repos to https
XServer Deploy tasks.
Setup apparmor for xserver.
Add a section for the xserver.
Add the xserver upstart script.
Move git install to common.
Correct version variable name.
Create a virtualenv for the python sandbox.
Update apparmor configurations.

Allow for the app armor configuration files to be loaded from secure repo if
available and loaded from the base configuration otherwise.
Update jinja templating.
Add nginx tasks to xserver.
Update nginx port configurations.
Add a task for installing xserver requirements.
Correct the pip install settings and add system packages needed by numpy.
Change the mode on the sandbox sudoers file.
Move xserver vars into a map.
Switch to dot notation instead of bracket notation.
Move back to prefix notation for xserver vars.
Add xserver env file and checkout the grader code.