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.
Is `project_member` too similar to `project_memberships`? If not, I'll change it.

My thinking was "assignable" is something that *can be assigned a role*, where as "member" actually implies the role exists. The very term "member" implies membership, but there isn't any in this context.