Automatic merge from submit-queue (batch tested with PRs 58300, 58530, 57942, 58543). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Ability to specify OS_* variables for OpenStack configuration

**What this PR does / why we need it**:

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.


**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
Authentication information for OpenStack cloud provider can now be specified as environment variables
```