Make db utils importable without migrate

One of functions defined in openstack.common.db.sqlalchemy.utils uses
UniqueConstraint class imported from migrate (sqlalchemy-migrate). It
actually makes sense to move import of migrate directly into the
function to make utils module importable in projects which use
alembic instead of sqlalchemy-migrate (e.g. neutron and ironic).

Change-Id: I382f8c14c69050c8f2ea0a0ba6bf2f1eef84e14d
Merge "Make db utils importable without migrate"