OS-432 Update existing NIOS network on subnet create

In case
 a. user creates a subnet for shared or external network
 b. NIOS network with the same CIDR exists
Update existing network, rather than raising an error.

Change-Id: If9518c6fd7e6a18943c959339cfca72a504b8ee1
The only thing changed that I see here is that the `allocate_specific_ip` method now returns IP address or `None`, the rest of the changes are simple renaming and shuffling variables around. Not sure if it's really needed.