Config methods for reserving/releasing IPs

Use TemporaryReservationManager for IP handling to ensure config data
consistency.

Implement:
 - methods for reserving/releasing IPs
 - _UnlockedCommitTemporaryIps()
 - CheckIPInNodeGroup() used for conflicting IPs
 - GetGroupNetParams() used for inheriting netparams.

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