Expose the vendor_passthru resource

The vendor_passthru resource allow vendors to expose a custom
functionality in the Ironic API. Ironic will merely relay the message
from here to the appropriate driver, no introspection will be made in
the message body.

Change-Id: I81f6460da80d44ab879fa4cadfbc0021b38ec96f
Merge "Expose the vendor_passthru resource"