Fix bug 2302: IP ranges management in Azure not working properly

OSM uses a specific vnet to create NS or VNF subnets when instantiating on
Azure. The vnets allow more than one address prefix, but RO only try to
allocate new subnets in the range defined in first place, ignoring the rest.

Change-Id: I9792caf2a468e60b6762567a9b4a90d0f1646b39
Signed-off-by: aguilard <e.dah.tid@telefonica.com>