Additional image member information

Return the created_at and updated_at values for image member.

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

Change-Id: I50f514c8680d1299d41c107c993e7ee2b0d7a185

Image Member Domain Model

This commit is the first layer of image member domain model.
Images and Image Members share a two way relationship. Where
an image can have a list of members who can access the image,
and a member can have a list of images which be accessed by
the member. The can_share feature from v1 of glance api
will not be ported to glance v2 api.

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

Change-Id: Ibfb790d438efb0f5b29b4786c02f1155962150c5

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.
For a given image member, a user can get a list of images
corresponding to that image member.
Also, this commit issues the bug in glance v1 api of image members,
and does not allow a user to view the private images of the member,
unless the user has the right access permissions.

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

Change-Id: I71a0c0d3c828cba231a75d0b7888bad932de41e5

Adding image members in glance v2 api

Adding image members to glance v2 api. A user can create image member,
list image members and delete image member for a given image.
A user can also view the list of images for a given image member.
This is the prework for image sharing feature.

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

Change-Id: Ifc0607d693f7b0218ebd7fc4824a64e9eba995b3

Adding Tenant class

Change-Id: I0faa6338304091b9c49fb65900cb0f6ef60b012d