[KNIFE-423] Add new command 'knife openstack network list', add option --network-ids...

https://tickets.opscode.com/browse/KNIFE-423

This patch I am proposing will add two pieces of functionality. First a new command "knife openstack network list" will return a list of available networks. Second adding a command to the knife openstack server create command that will allow you to specify which network(s) you wish to apply to your new instance.