webadmin: Re-assign roles when network is detached

When a network is detached from a cluster, we make sure to visibly
re-assign those roles to the management network.

Change-Id: I72a9b4a46a32014b31f8981fbe0e8750352dd80e
Bug-Url: https://bugzilla.redhat.com/1135785
Signed-off-by: Lior Vernia <lvernia@redhat.com>