Add driver_validate()

vendor.driver_vendor_passthru() should be validated before called
on Ironic conductor, like vendor_passthru(). Add a new method
driver_validate() to support this.

Change-Id: Idff802cece01940b8ade34fa343f1bc8e76f1630
Closes-Bug: #1391580
Support configdrive in agent driver

This passes the configdrive URL to the agent, to be downloaded
and written to a partition during deployment.

Implements: blueprint expose-configdrive

Change-Id: I3d142acb76a87d92a3f7b0be039c157cda4a7aa8
Merge "Add driver_validate()"
Merge "Support configdrive in agent driver"