Add os_auth_url to metadata; document provisioning patterns

Adds the OpenStack auth URL to the instance's metadata to
make many provisioning configuration tasks easier and more
portable.

Additionally adds a new section to the README documenting
how to get a number of commonly-needed parameters for
provisioning which are not otherwise obvious.
Merge pull request #124 from gabrielhurley/auth-url-metadata

Add os_auth_url to metadata; document provisioning patterns