Bug 1185643: use in-tree mozharness for builds; r=wander

This required moving the custom mozharness configs out of the builder image
(which puts them in ~, where mozharness no longer looks) and moving them into
mozharness itself.

testing/taskcluster/mach_commands.py

@@ -283,6 +283,8 @@ class Graph(object):
         jobs = templates.load(job_path, {})
 
         job_graph = parse_commit(message, jobs)
+
+        # once everything uses in-tree mozharness (bug 1187706), this can go away.
         mozharness = load_mozharness_info()
 
         # Template parameters used when expanding the graph