Include DiskFilter in the default list

DiskFilter is pretty critical for proper operation of a deployment without
shared storage, and is likely to be something most new operators will want
enabled. We should default to include this.

DocImpact: The DiskFilter is now included in the default set

Change-Id: Iefa5538a58a61986c466912739408ee291cdc6c2

nova/scheduler/host_manager.py

@@ -56,6 +56,7 @@ host_manager_opts = [
                   'RetryFilter',
                   'AvailabilityZoneFilter',
                   'RamFilter',
+                  'DiskFilter',
                   'ComputeFilter',
                   'ComputeCapabilitiesFilter',
                   'ImagePropertiesFilter',

nova/tests/unit/scheduler/test_caching_scheduler.py

@@ -105,6 +105,7 @@ class CachingSchedulerTestCase(test_scheduler.SchedulerTestCase):
             "root_gb": 1,
             "ephemeral_gb": 1,
             "vcpus": 1,
+            "swap": 0,
         }
         instance_properties = {
             "os_type": "linux",
@@ -127,6 +128,7 @@ class CachingSchedulerTestCase(test_scheduler.SchedulerTestCase):
             'host_%s' % index,
             'node_%s' % index)
         host_state.free_ram_mb = 50000
+        host_state.free_disk_mb = 4096
         host_state.service = {
             "disabled": False,
             "updated_at": timeutils.utcnow(),