Specify correct constraint name for postgresql

The UniqueConstraint is not named 'name,' this causes postgres to fail
with the following error:
UTC ERROR: constraint "name" of relation "datastore_versions" does not
exist
The constraint name should be 'datastore_versions_name_key' in the case
of postgresql.

Since postgres isn't tested in trove's gate yet (Fix:
I53e7fa2729542b88b43cd3de51e15118adaeec57) this has been tested locally
to make sure it stack.sh (which runs trove-manage db_sync) works.

Co-Authored-By: Nikhil Manchanda <SlickNik@gmail.com>
Change-Id: I265925566b8e84d92c838cb3b39ad1f8502cd052
Related-Bug: #1321093