Multiple openstack networks support and switch to select floating ip network

This MR adds support to multiple openstack networks and select one of those networks as floating ip network (defaults to first). Our use case is to use this version of docker-machine to auto scale gitlab runners in private openstack environment. For example vSphere driver supports similar case.

Your template mentions that repository will be maintained only for bug fixes affecting running costs: I think this MR is in gray area since it reduces running costs via enabling autoscale for certain Openstack environments but this is not actually bug fix. For us it also reduces maintenance costs since we could just your binaries instead of keeping our own fork. :D

Feel free to close this MR if it isn't in your scope.