Introduce client support for networks

gnt-network is used to manipulate and handle networks that
currently provides the following operations:

 * Add a new network:
   gnt-network add --network=1.2.3.0/28 --gateway=1.2.3.1
                   --add-reserved-ips=1.2.3.4,1.2.3.5 testnet

 * Remove an existing network:
   gnt-network remove testnet

 * Modify an existing network:
   gnt-network modify --gateway=1.2.3.6
                      --network-type=private
                      --network6=2001:648::/64
                      --gateway6=none testnet
                      --add-reserved-ips=1.2.3.10,1.2.3.10,
                      --remove-reserved-ips=1.2.3.20
                      testnet

 * Connect an existing network to a nodegroup:
   gnt-network connect testnet default bridged br100
   gnt-network connect testnet <nodegroup> <mode> <link>
   (pass all for <nodegroup> to connect to all nodegroups)

 * Disconnect an existing network from a nodegroup:
   gnt-network disconnect testnet <nodegroup>
   (pass all for <nodegroup> to disconnect from all nodegroups)

 * List available networks:
   gnt-network list

 * Show network info:
   gnt-network info [testnet]

Introduce new option NOCONFLICTSCHECK_OPT for not checking for
conflicting IPs. Using this might cause data inconsistency.

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