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
Not sure about this, lets do it in sepparate patch.