Allowing member to set status of image membership

Only the owner of an image can add members to the image.
By default the status of the image member is requested.
The member can change the status of the image membership
to accepted or rejected. This is done to prevent spamming
the user's image list. A member cannot see the other members
of the image.

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

Change-Id: I0d0deba4b0df52b2f8d105b779fb7de746229d3a
Merge "Allowing member to set status of image membership"