Import fixes from the Nova driver reviews

Import fixes from the Nova driver review, based upon revision 7
of https://review.openstack.org/#/c/103167/

- better use of _L* helpers
- dynamically import python-ironicclient when the driver is initialized,
  so that Nova does not need to add it to requirements.txt
- remove unused CONF options pxe_bootfile_name
  and instance_type_extra_specs

Co-authored-by: Devananda van der Veen <devananda.vdv@gmail.com>

Change-Id: Iee1481632e9927766776c7d35264e8cf4c469c8a
Merge "Import fixes from the Nova driver reviews"