The ability to load the scheduler_host_manager from a random
classpath was deprecated in Mitaka. This removes that ability
so that only the entrypoints in Nova's setup.cfg can be used
to load the scheduler host manager.
The scheduler_host_manager config option is updated to only
allow the choices that are currently in the setup.cfg.
And a release note is added for the upgrade impact of dropping
the deprecated classpath loading ability.
Change-Id: I886c74afac02d100d472b78522e8295748276e4d
Related-Bug: #1403020
7e2f5c7d34