openstack: Document the two platforms

[pierreprinetti]

As detailed in https://github.com/coreos/afterburn/pull/462, Afterburn has two separate OpenStack endpoints, each with a different metadata gathering policy. This patch adds the relevant information to the docs.

[bgilbert]

I agree that this document is probably the best place for this info, but the current PR draft adds it as an odd parenthetical and only for two of the providers. I'd suggest something like this:

• openstack
  • Metadata source: configuration drive if present, otherwise metadata service
  • Features:
    • Attributes
    • SSH keys

and ideally the PR would add that info for all providers.

[pierreprinetti]

Reformatted as requested. PTAL!