Change conductor live migrate task to use select_destinations()

Live migration currently makes calls to scheduler select_hosts() to select
a suitable target host if none is given. Other tasks instead queries the
scheduler by making calls to select_destinations(). Since the latter call
basically returns the same result, and because live migration is the only
task using select_hosts(), changing to select_destionations() would mean,
the scheduler would no longer need the select_hosts() call.
Deprecate scheduler select_hosts()

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