Deprecate scheduler select_hosts()

This call is no longer used and can be removed from the scheduler api.

Conductor manager, as the last user of this call, used it to select
a target for live migration, but now instead makes a call to
select_destinations() to get the same information. The related change
to conductor manager is here: https://review.openstack.org/#/c/52311/

Change-Id: I702515a086e7f33c48fa9ee106271c868b660540