Avoid having to configure the full class path of scheduler driver,
change to load by stevedore driver plugin using entrypoints.
Change 'scheduler_driver' to use entrypoint with the namespace
'nova.scheduler.driver' in 'setup.cfg'. Meanwhile, still maintain the
compatibility for class path configuration until the next major release.
Change all related tests with flag 'scheduler_driver' to use stevedore
entrypoint.
UpgradeImpact - see the reno file attached.
Implements: blueprint scheduler-driver-use-stevedore
Change-Id: I8c169e12d9bfacdbdb1dadf68b8a1fa98c5ea5bc