PXE validate() to fail if no Ironic API URL

As part of the deploy process the conductor should pass the URL of the
Ironic API to the deploy ramdisk so that it can talk back to the Ironic
API, but if the URL is not present in the Ironic configuration file or
in the keystone catalog the deploy process will fail in the middle. This
patch adds a verification to the validate() method to make sure the URL
is present before the deploy starts.

Related-Bug: #1290482
Change-Id: I95472660deff0e23a19d0b744f711bbe3acc2191
Merge "PXE validate() to fail if no Ironic API URL"