Re-enable a lot of cinder scheduler tests

This highlight and fixes a NoneType exception in the schedulers
_get_weighted_candidates method

This was originally added to get around the fact that the filter tests
required that cinder was correctly installed on the system. But the
is_cinder_installed condition always returning False and this hid
problems of old packages been installed in our system. Also
930f5891b0 added the setup.py automatically
to run_tests.sh so we can remove this.

See https://review.openstack.org/#/c/20213/ for history.

Fixes bug: 1213226

Change-Id: I86fee802c0543355471ddbd712e52ccec750cea0

cinder/scheduler/filter_scheduler.py

@@ -222,7 +222,7 @@ class FilterScheduler(driver.Scheduler):
         hosts = self.host_manager.get_filtered_hosts(hosts,
                                                      filter_properties)
         if not hosts:
-            return None
+            return []
 
         LOG.debug(_("Filtered %s") % hosts)
         # weighted_host = WeightedHost() ... the best