Keystone v3 improvements.

Managers for project group membership, project user membership, project combined membership (users and groups); for group project membership, group users, group roles; for user project membership, user roles.
In this case, anything which would be part of the "project_assignables" list/get calls must have a membership relation (a role) because otherwise it wouldn't be listed. So I think calling these "project_members" and the roles themselves "project_memberships" is fine. The memberships and the members themselves *are* different things.

For example, if you subscribe to a gym, your membership is not the same thing as you the person who goes to the gym. I'd like to make this change.