Fix UpdateProjectMembersAction excess keystone calls

Patch adds a single call to keystone user_list with a specified project field
in place of polling all users roles to find out if they have any roles in the
current project.

Change-Id: Ia304af05cdb5be13e67762e0a0a44b17824c30bf
Fixes: bug #1224114
Merge "Fix UpdateProjectMembersAction excess keystone calls"