This commit updates the support for pushing images into OpenStack by
inheriting environment variables for endpoint and authentication
information, when available.
It also attempts to make the `openstack run` support more consistent
with other providers (specifically GCP and AWS), i.e just take the name
of the image as the argument and launch an instance using that.
Finally, it also updates the relevant documentation for OpenStack
support.
Signed-off-by: Nick Jones <nick@dischord.org>