Support env variables for more arguments
Support env variables for more arguments

Magnum team is try to integrate k8s-keystone-auth when creating
k8s cluster so that user can get keystone authN and authZ for k8s
out of box. However, Magnum is not running kubelet on master(for
flannel network driver) or doesn't allow running workingload on
master(for calico network driver). So we need a good way (env
variables) to pass in those necessary arguments. This patch adds
the env variables support for those must-have arguments.

kubernetes/cloud-provider-openstack,kubernetes/cloud-provider-openstack,kubernetes/cloud-provider-openstack