switch instance_discovery to libvirt_metadata

Since Gnocchi is the recommended backend, and other are deprecated.
We can switch instance_discovery to the libvirt_metadata.

Change-Id: Ib846b87bfb76c359fdec28d6cd258781fd201348

ceilometer/compute/discovery.py

@@ -41,7 +41,7 @@ OPTS = [
                      'compute agents to be run simultaneously. '
                      '(replaced by instance_discovery_method)'),
     cfg.StrOpt('instance_discovery_method',
-               default='naive',
+               default='libvirt_metadata',
                choices=['naive', 'workload_partitioning', 'libvirt_metadata'],
                help="Ceilometer offers many methods to discover the instance"
                     "running on a compute node: \n"

ceilometer/tests/unit/compute/test_discovery.py

@@ -124,6 +124,9 @@ class TestDiscovery(base.BaseTestCase):
         self.CONF.set_override('host', 'test')
 
     def test_normal_discovery(self):
+        self.CONF.set_override("instance_discovery_method",
+                               "naive",
+                               group="compute")
         dsc = discovery.InstanceDiscovery(self.CONF)
         resources = dsc.discover(mock.MagicMock())
 
@@ -140,6 +143,9 @@ class TestDiscovery(base.BaseTestCase):
             self.CONF.host, "2016-01-01T00:00:00+00:00")
 
     def test_discovery_with_resource_update_interval(self):
+        self.CONF.set_override("instance_discovery_method",
+                               "naive",
+                               group="compute")
         self.CONF.set_override("resource_update_interval", 600,
                                group="compute")
         dsc = discovery.InstanceDiscovery(self.CONF)

devstack/plugin.sh

@@ -243,7 +243,6 @@ function _ceilometer_configure_storage_backend {
         iniset $CEILOMETER_CONF DEFAULT meter_dispatchers database
         iniset $CEILOMETER_CONF database metering_connection mongodb://localhost:27017/ceilometer
     elif [ "$CEILOMETER_BACKEND" = 'gnocchi' ] ; then
-        iniset $CEILOMETER_CONF compute instance_discovery_method libvirt_metadata
         iniset $CEILOMETER_CONF DEFAULT meter_dispatchers gnocchi
         iniset $CEILOMETER_CONF DEFAULT event_dispatchers gnocchi
         # NOTE(gordc): set higher retry in case gnocchi is started after ceilometer on a slow machine

releasenotes/notes/instance-discovery-new-default-7f9b451a515dddf4.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    Ceilometer legacy backends and Ceilometer API are now deprecated. Polling
+    all nova instances from compute agent is no more required with Gnocchi. So
+    we switch the [compute]instance_discovery_method to libvirt_metadata.
+    To switch back to the old deprecated behavior you can set it back to 'naive'.