multiple assignment bug

when I want to assign one single node to the roles `neutron-server` and `neutron-l3`,
I can do that if `neutron-server` is assigned **first**.
but if `neutron-l3` is assigned first, and i drag and drop the same node to `neutron-server`, I get the following exception:
`Failed to assign network to neutron-server, it's already assigned to another role`
so this patch makes multiple role assignment possible, when neutron-l3 is assigned before neutron-server.