When we convert the OpenStack cloud provider to run in an external
process, we should be able to use kubernetes Secrets capability to
inject the OS_* variables. This way we can specify the cloud
configuration as a configmap, specify secrets for the userid/password
information. The configmap can be mounted as a file. the secrets can
be made available as environment variables. the external controller
itself can run as a pod/daemonset.

For backward compat, we preload all the OS_* variables, if anything
is in the config file, then that overrides the environment variables.