Prevent the user from creating a single IP address sized network

If the user doesn't specify a mask when creating a subnet, the mask
defaults to /32 (IPv4) or /128 (IPv6) making it impossible to allocate
an IP address when later launching an instance. This patch prevents
creating a subnet with such a small mask.

Fixes bug #1102504

Change-Id: I4962e0a416b8ec9aca013e41783994f65c22a989