We set global variables in Ironic devstack plugin like
DEFAULT_IMAGE_NAME. Ensure that grenade uses them by
sourcing lib/ironic from upgrade/settings.
Set IRONIC_DEPLOY_DRIVER by default to pxe_ipmitool. As we no longer
add IRONIC_DEPLOY_DRIVER to IRONIC_ENABLED_DRIVERS and defaults are
conflicting.
Closes-Bug: #1663371
Change-Id: I462ac792ad7920beff64c15a7c005406fcfa5cce