Make sure domains are enabled by default

Domains without an explicit 'enabled' key in the dict will now be
handled in the same way as projects are. This will ensure that
the KVS backend produces the same result as the SQL backend for
assignment.

Change-Id: I3b6d26278329841cfe1be5ce7f67ba6d3569785f
Related-Bug: #1311142
Merge "Make sure domains are enabled by default" into stable/icehouse