Introduce new module for IP pool management

Add new library module lib/network.py.
Introduce new class: AddressPool.

AddressPool implements all operations needed for managing IPs inside
the IP pool.

Given a Network config object (nobj), the class:

 * initializes the corresponding IP pool object via
   network.AddressPool.InitializeNetwork(nobj)
 * obtains the corresponding IP pool object via
   network.AddressPool(nobj)
 * manipulates IPs inside the pool

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