Devstack: Use [deploy] erase_devices_priority config option
Devstack: Use [deploy] erase_devices_priority config option

The configuration option [agent] agent_erase_devices_priority was
deprecated and renamed to [deploy] erase_devices_priority as part of the
commit 9a35ca19b4872af2e02cb83b21a3746732802e74.

This patch also fixes the FIXME comment because it's wrong, cleaning is
not disabled in gate, only the erase_devices step.

Change-Id: Iae25a5c672896af77fdd2005371d2515aabaa830

bacaldwell/ironic,pshchelo/ironic,openstack/ironic,devananda/ironic,ionutbalutoiu/ironic,SauloAislan/ironic,SauloAislan/ironic,dims/ironic,dims/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,openstack/ironic,NaohiroTamura/ironic,pshchelo/ironic,NaohiroTamura/ironic