After Ironic is moved to devstack plugin the tempest is always
executed before ironic devstack plugin. Unfortunatly there is no quick
way to change order.
This change introduce new function ironic_configure_tempest that is
called from Ironic's devstack plugin. It updated flavor_ref and
flavor_ref_alt compute tempest options.
Change-Id: I85a77dd2193bac0ea4bd4fc1985c82d9f598b06b