Document 'POST /v1/.../vendor_passthru'

http://docs.openstack.org/developer/ironic/webapi/v1.html does
not contain anything about how to use vendor passthru endpoints.
This adds 'POST /v1/nodes/(node_uuid)/vendor_passthru' and
'POST /v1/drivers/(driver_name)/vendor_passthru' to that
html page.

Change-Id: I6191be759f168c58e80d946530af7a0c4267501c
Closes-Bug: #1331415
Merge "Document 'POST /v1/.../vendor_passthru'"