New Feature: Gateway appliance management 

`sl gateways` would be the command group

- `sl gateway list` -> https://cloud.ibm.com/netsec/gateway-appliances
- `sl gateway detail` -> https://cloud.ibm.com/netsec/gateway-appliances/620574/details#overview
- `sl gateway vlans` -> https://cloud.ibm.com/netsec/gateway-appliances/620574/details#vlans

https://github.ibm.com/SoftLayer/softlayer-cli/issues/773