There are a few places in unit tests where we rely on Redfish constants
having well-known values. Use constants themselves instead.
An even more problematic place is _set_boot_device: it relies on storing
Redfish constants in driver_internal_info. With the future switch to
enums, it will no longer be possible. Use Ironic constants and provide a
compatibility layer.
Change-Id: I8e075f808e013afd778dd7cdc0927fe339309b70
93fd147b6f