Add config objects and methods for IP pools

This patch introduces the following changes to lib.config and lib.objects to
facilitate IP pool management.

- Add a "networks" key to objects.cluster to hold link -> subnet relations.
  Each link is assumed to be associated with at most one IPv4 and at most one
  IPv6 subnet. Currently only IPv4 subnets are used.
- Add a TemporaryReservationManager for IP address reservations.
- Add config.{_UnlockedCommitIp,CommitIp,_UnlockedReleaseIp,ReleaseIp} to write
  IP pool changes to the config file.
- Add config.{ReserveIp,GenerateIp} for use with the _temporary_ips
  TemporaryReservationManager.
- Commit succesful reservations/releases to the IP pools in config.AddInstance
  and config.RemoveInstance

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>