Use abstract base class for identity driver

Use the abc module for managing abstract base classes in the driver
layer.

bp abstract-base-class-drivers

Change-Id: Ifc3b7c158a3a26ca0295e5e133d0108a357b1e37
Remove unused member

keystone.assignment.backends.ldap:Assignment._identity_api was not
used, so remove it.

Change-Id: If4dc5a47c87e9aaf97e3db20a9b813ba32754bea
Merge "Use abstract base class for identity driver"
Merge "Remove unused member"