Added mixin class to allow soft deletes
Added mixin class to allow soft deletes

Modified the base model class to do hard deletes by default,
and added a mixin to allow for soft deletes.

Change-Id: I62987b8752b1dd53fd20d475f8913f40836aac0d
Implements: blueprint identify-cas

openstack/barbican,MCDong/barbican,openstack/barbican,cloudkeep/barbican,cneill/barbican,cneill/barbican,MCDong/barbican,cloudkeep/barbican