Modify LUInstanceCreate to support networks

Implement backend support, to export the IP pool management
functionality to the clients. When the new NIC parameter 'network' is
given, the ippool management system is triggered.  If a NIC belongs to
a network, it inherits the netparams (mode, link) as its nicparams. If
a network is requested, then no 'mode' or 'link' should be
specified. Backwards compatibility is preserved, if 'network'
parameter is omitted.

Examples for IP pool management system:

--net 0:network=net1
  for an IPless NIC in a network

--net 0:ip=pool,network=net1
  for automatically generated IP inside the network

--net 0:ip=1.2.3.4,network=net1
  for specific IP inside the network

--net 0:network=none
  for IPless NIC with default nicparams

Examples for traditional assignment:

--net 0:ip=1.2.3.4,link=br100,mode=bridged
--net 0:ip=none,link=rt2,mode=routed
--net 0

Check for conflicting IPs to ensure IP uniqueness inside nodegroups.

IAllocator should be extended to become network aware. If an instance
has a NIC inside a network, then IAllocator must pick a node that its
nodegroup is already connected with that network.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Iustin Pop <iustin@google.com>