webadmin: adding migration role to a network.

Just one network in a cluster can have this role.

The effected dialogs are-
 Migration column was added-
   Network->Cluster->Assign/Unassing
   Cluster->Network->Assign/Unassign
 Migration icon was added to the roles icons on network panel and tooltip
   Network->Host->SetupNetworks
   Host->Network Interfaces->SetupNetworks

The effected sub-tabs are-
 The role column was update to show also migration network
   Network->Cluster
   Cluster->Network

Change-Id: I9752b182fd15f0557575ebe7e0271f6dd7a916f9
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>