Metadef ORM classes should use a common delete

[Wayne]

The metadef db delete functions typically have code like this:
session.delete(db_rec)
session.flush()

To be consistent with the ORM classes save() function implementation, it would be better to add a delete function
to the base metadef ORM class (i.e., GlanceMetadefBase).

Background - Both GlanceBase and GlanceMetadefBase ORM classes call oslo.db.sqlalchemy.models.ModelBase.save() for create/update.
There is no function for hard deletes in ModelBase.

The ModelBase.save function is simply:
def save(self, session)
    with session.begin(subtransactions=True):
        session.add(self)
        session.flush()
Note: See comments in ModelBase.save for more details on the need for "with session.begin(subtransactions=True_"

The new GlanceMetadefBase.delete() function should be implemented in a way that is consistent with the oslo ModelBase.save function implementation.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/122910

[OpenStack Infra (hudson-openstack)]

Change abandoned by Glance Bot (<email address hidden>) on branch: master
Review: https://review.openstack.org/122910