Add deprecation warning to nova baremetal virt driver

We have warnings in the other group C virt drivers and for cells, and
given the baremetal driver is deprecated and slated to be replaced by
the ironic driver, add a deprecation warning to the logs when the
baremetal driver is loaded.

Change-Id: Iaee8b7f7b4d32605e36fd72e893794066727fb65
Merge "Add deprecation warning to nova baremetal virt driver"