Allow users to specify ce-dev subnet

[gregharvey]

At the moment ce-dev assumes the default subnet is 172.17.0.0/24 and checks to see if there's a 172.18.0.0/24 ... if there is it tries 172.19.0.0/24 and so on, up to 30 to find a subnet for ce-dev to use.

It would be handy if that subnet could be set, e.g you could pass it a CIDR block and it would set up the ce_dev network accordingly, rather than trying to determine the next available subnet.