CLOUDSTACK-4023: lock the network during the shutdownNetwork (just the way we do when implement network)

Conflicts:
	server/src/com/cloud/network/NetworkServiceImpl.java
	server/src/com/cloud/network/router/VirtualNetworkApplianceManagerImpl.java
Create Private Network Test

Signed-off-by: Chiradeep Vittal <chiradeep@apache.org>