* Default value is 'aws' (unchanged)
* Used by EDXAPP_SETTINGS, gitreload_env.DJANGO_SETTINGS, and demo_edxapp_settings variables

If a git version is specified as a branch, and the commit referred to is
already on the local working tree, but the branch is not, then the
checkout will fail.

This is because the Ansible 1.9.3 module would check to see if the
commit was already present, and if so, did not fetch. This meant the
branch wasn't defined in the local working tree.

This can happen if you make a branch but there aren't any changes on the
branch.

Also support updating repositories such as python graders
from xqwatcher/xserver