Implement LUs for corresponding opcodes:
 * LUNetworkAdd:
   - Check for IP validity
   - Reserves all necessary IPs
   - Create new Network config object
 * LUNetworkRemove:
   - Checks if connected to any nodegroup
   - Remove a Network config object

Implement basic config methods:
 * LookupNetwork()
   - Given the network name return the network UUID
 * AddNetwork()
   - Add a new network to the config
 * RemoveNetwork()
   - Remove a network from the config

Add new locking level: LEVEL_NETWORK

Add various useful config methods for retrieving network info.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>