Avoid having to configure the full class path of host manager using
classloader. Change to load the class by stevedore driver plugin using
entrypoints.
Change 'scheduler_host_manager' to use entrypoint with the namespace
'nova.scheduler.host_manager' in 'setup.cfg'. Meanwhile, still maintain
the compatibility for class path configuration until the next major
release.
UpgradeImpact - see the reno file attached.
Change-Id: I3fd42ead44487a21eb5cfaf5a91209277ce30ad0
Partially-Implements: blueprint scheduler-driver-use-stevedore