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>