Improve temporary reservation of IP addresses

Add a new method, TemporaryReservationManager.GetECReserved(), which returns all
reservations held by a given ec_id.

Also add a new method, ConfigWriter._UnlockedCommitReservedIps(), which can be
called to commit all temporarily reserved IP addresses to the respective
address pools. For this to work, the IP reservation manager must hold both, the
address and the network UUID of the respective address pool.

Finally, we automatically commit IP address reservations during
ConfigWriter.Update() of Instance objects. For this to work, we add an optional
ec_id keyword, which must hold the calling job's unique id to get the
relevant reservations.

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