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
Merge "Adding database layer for image members domain model"