Fix reservation_commit so it works w/ PostgreSQL.

Update the Reservation model so that the 'usage' relationship is
explicitly forced to use an inner join. This fixes an issue on
PostgreSQL which doesn't support 'SELECT FOR UPDATE' on outer
joined queries.

Fixes LP Bug #1003756.

Change-Id: I3c40bd8481855a18391e12d7411762e91b1ef8b0

nova/db/sqlalchemy/models.py

@@ -465,9 +465,11 @@ class Reservation(BASE, NovaBase):
     uuid = Column(String(36), nullable=False)
 
     usage_id = Column(Integer, ForeignKey('quota_usages.id'), nullable=False)
+    # NOTE(dprince): Force innerjoin below for lockmode update on PostgreSQL
     usage = relationship(QuotaUsage,
                          backref=backref('reservations'),
                          foreign_keys=usage_id,
+                         innerjoin=True,
                          primaryjoin='and_('
                               'Reservation.usage_id == QuotaUsage.id,'
                               'Reservation.deleted == False)')