We broke the possibility in Ocata with Icdcf839b6d28893694bfa1355e9dbe8dbb5ea8c3
to use other scheduler drivers but the ones we provided in tree.
Unfortunately, that was an incidental change without any communication.
Removing the choices kwarg will allow operators to run their own scheduler driver.
Whether Nova would stop supporting custom drivers would require a totally separate
change which would clearly communicate thru a deprecation notice but that is not
the intent for that bugfix, which aims only to bring back the capability.
Change-Id: I346881bc3bc48794b139cc471be1de11c49b8ee3
Closes-Bug: #1704788
1e5c7b52a4