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: Dimitris Aragiorgis <dimara@grnet.gr>