Addition of gitreload role for github Web hook course imports

It also supports updating repositories such as python graders
from xqwatcher/xserver.
That is because that folder is set in edx-platform settings file so this syncs it with the default value there.  I'll go ahead define that setting in the edxapp role and reference it here instead of using the default.  The edxapp dependency isn't in meta though because this app gets used in different ways, and only one way is dependent on edxapp.  The first is importing courses into platform, and the other is just updating repos (we use it for xserver graders for example, but there are other utility uses).  To solve that I include edxapp role vars in tasks: https://github.com/edx/configuration/pull/1679/files#diff-036ed9abea637f04ba8f9b5665ded842R35