Add the --no-mergepy option

Passing `--no-mergepy` to devtest or devtest_overcloud, will use the
standalone templates (overcloud-without-mergepy.yaml and friends) instead of
building overcloud.yaml by running merge.py on overcloud-source.yaml.

Since the standalone templates rely on a resource registry specified in a
separate Heat environment file (overcloud-resource-registry.yaml), this feature
requires python-heatclient version 0.2.11 which added support for passing
multiple environment files to `heat`.

Change-Id: Ib82eecf0998ae5c0a811a248b638c5633c8bb262
Merge "Add the --no-mergepy option"