Ensure metadata access network does not prevent router deletion

Bug 1152136

This patch checks if a metadata access network is still present when
removing a router in the NVP plugin. If yes, the network is detached
from the router and then destroyed, thus ensuring a safe completion of
the operation as well as removal of metadata access network resources.

Change-Id: Id8be6ec50ab4536f4ae9916dadaed56644597c7c
Merge "Ensure metadata access network does not prevent router deletion"