Add vendor_passthru method to RPC API

Method vendor_passthru allows Ironic API
service validate and to pass vendor-specific
info to a node driver.

Partially implements blueprint pxe-mount-and-dd

Change-Id: I2c60d84e9d983a301c9169f3a1e9a34e93d2b050
Merge "Add vendor_passthru method to RPC API"