As part of the docs migration from openstack-manuals to
nova in the pike release we missed the config-drive docs.
This change does the following:
1. Imports the config-drive doc into the user guide.
2. Fixes a broken link to the metadata service in the doc.
3. Removes a note about liberty being the current release.
4. Adds a link in the API reference parameters to actually
point at the document we have in tree now, which is
otherwise not very discoverable as the main index does
not link to this page (or the user index for that matter).
Partial-Bug: #1714017
Closes-Bug: #1720873
Change-Id: I1d54e1f5a1a94e9821efad99b7fa430bd8fece0a
59bd2f6adc