adding EDXAPP_LOCAL_GIT_IDENTITY to abbey.py

also adds some clarifying comments

E0d/es cluster

edxapp-git-auth: allow authenticated edx-platform/edx-theme checkouts

Using `EDXAPP_LOCAL_GIT_IDENTITY` gives parsing errors during evaluation for paths

Setting default username to root and defalut db name to edxapp.

Change how abby gets notified.

We were previously using the jenkins api which was more complicated than the
buildByTokens plugin and its API.

Jarv/focaccia

Feanil/mckinsey abbey

These vars are also set by the jenkins job

runs the worker role before the edxapp role

Remove hash_behaviour=merge

Because the CMS uses the git hash of the edx-platform for the
static path it can be problematic if you update the edx-platform
repo for the worker AFTER you update the edx-platform repo
for edxapp.  When running the edxapp role in worker mode
rake gather_assets is skipped.

This is a workaround but we may want to consider using separate
repos for worker and edxapp for situations where both are installed
on the same server.