Fix doc build errors in models.py

The docs build has errors like the following:
nova/nova/db/sqlalchemy/models.py:docstring of
nova.db.sqlalchemy.models.relationship:517: ERROR: Unknown interpreted
text role "paramref".

To fix this remove the import for relationship and refer to it
instead using orm.relationship.

Change-Id: I2e0a08438174f0fe0d7ea732670971e65811a700
Merge "Fix doc build errors in models.py"