Explicitly set Ironic's enabled drivers list

Because the "pxe_ssh" driver is not intended for production use,
it is disabled by default in Ironic by:
  https://review.openstack.org/88711

This patch explicitly enables both the pxe_ipmitool (default)
and pxe_ssh (used in testing) drivers for TripleO-based
testing environments.

Change-Id: Ide0f504445b8470b84b52ed6dbf63956b01854ce
Merge "Explicitly set Ironic's enabled drivers list"