Add option to Magnum to use the new external cloud provider for openstack

[Davanum Srinivas (DIMS)]

There is a process going on to remove cloud provider code from kubernetes main repository:
https://github.com/kubernetes/community/blob/master/keps/0002-controller-manager.md

Essentially any of the kubernetes binaries that used --cloud-config=openstack should now use --cloud-config=external and run another binary that has some of the controllers.

Please see the new repository for openstack that has a bunch of new binaries/manifests etc:
https://github.com/kubernetes/cloud-provider-openstack

thanks,
Dims

[Davanum Srinivas (DIMS) (dims-v)]

https://github.com/kubernetes/cloud-provider-openstack/issues/48