Improve handling of empty resource_registry
Improve handling of empty resource_registry

It's possible that content["resource_registry"] might
not exist or be iterable so wrap the reference to it
with a try/except for a KeyError or TypeError and continue
the deployment without the empty resource_registry instead
of failing.

Change-Id: Ic7d88382a0ff15c24800f12ddbf69f13d6804b28
Closes-Bug: #1833743

openstack/python-tripleoclient