Adding database layer for image members domain model

Adding the database layer image members domain model.
For a given image, a user can get a list of image members,
delete image member, and get a specific image.

Related to bp glance-api-v2-image-sharing

Change-Id: I71a0c0d3c828cba231a75d0b7888bad932de41e5
Fix moker typo in test_notifier

test_notifier.TestQpidNotifier are usually skipped, so
that typo was not detected while running unit tests.

Change-Id: I1998b8e72c20170a8a2b234f2c19ec1e990ed43e
Merge "Adding database layer for image members domain model"
Merge "Fix moker typo in test_notifier"