Fix doc build errors in db/sqlalchemy/models.py

Building the documentation creates errors like:
manila/db/sqlalchemy/models.py:docstring of
manila.db.sqlalchemy.models.relationship:53: ERROR: Unknown interpreted
text role "paramref".

Fixed with changing the import of relationship to orm.relationship.

Change-Id: I51132581c2bf6a4e68a945a972401dbaed517733
Closes-Bug: #1371739
Merge "Fix doc build errors in db/sqlalchemy/models.py"