Fixes bug 1043147
Currently, the cisco nexus sub-plugin doesn’t handle the network admin state parameter
when it’s updated. The fix would enable the sub-plugin to handle an update of the admin
state of the network. In update_network of the nexus sub-plugin, vlan corresponding to that network
is added/removed from the nexus interfaces based on the network state. Unit tests are added as well.
Change-Id: Ia3b5400c2896c1b0160968a2c75c21247ca0c3f4