Refactors scheduler.chance.select_hosts to raise NoValidHost

This change is intended to refactor the method to be consistent
with the filter_scheduler.select_hosts implementation.

This method is called by scheduler.manager and it is exposed
through the scheduler.rpcapi. Thus, both implementations should
behave the same.

This change is needed by others that will move cold migration
to conductor.

Partially implements bp cold-migrations-to-conductor

Change-Id: I0f6fef43324cf829cdb26633874c537572423f77
Merge "Refactors scheduler.chance.select_hosts to raise NoValidHost"