This change enforces usage of system scope credentials to manage
flavors, aggregates, and services, following the new policy rules for
SRBAC support in nova.
The logic to look up credential for the nova service user from
[keystone_authtoken] is left to keep backward compatibility but is
deprecated and will be removed.
Depends-on: https://review.opendev.org/806474
Depends-on: https://review.opendev.org/828025
Depends-on: https://review.opendev.org/828874
Change-Id: I71779f0f1459d64914589a94a440336386266306
0ed626e146