* Remove unused functions and imports from logic/subnets.py
* Remove --network-id option from snf-manage subnet-modify and make
it an argument
* Fix --ip-version, --dhcp and --gateway options in snf-manage
subnet-create
* Move some code from logic/subnets.py to api/subnets.py
* Refactor the way we handle the gateway IP
* s/slac/slaac/g