Addition of gitreload role for github Web hook course imports

It also supports updating repositories such as python graders
from xqwatcher/xserver.
It is the default in platform: https://github.com/edx/edx-platform/blob/master/lms/envs/aws.py#L251 and since that is a directory that the platform writes to and reads from in order to do imports of the courses, it made sense to leave it there.  i.e. the sysadmin dashboard git course import and management command use this directory regardless of gitreload being installed or not, so defaulting it somewhere else means adding gitreload as a dependency of edx-platform.