Subnets with prefix length 0 are invalid

This patch changes the API behaviour to return a 400 error
when a subnet with /0 prefix length is specified.

This kind of subnet hardly make any sense, and also cannot
possibly work when DHCP is enabled.

Change-Id: I8f822f14b91475dcf86ea44ee607013e61cbb6f7
Closes-Bug: #1362651
Merge "Subnets with prefix length 0 are invalid"