Enable POST samples API when gnocchi enabled

Currently, if we enabled gnocchi, the every API of meters, resources,
samples will be disabled and 410 response will be returned. The posting
samples API shouldn't be diabled.

Change-Id: I142e4470a69a0782cabb6b1b4160faac0d48f0df
Closes-Bug: #1505283

ceilometer/api/controllers/v2/root.py

@@ -156,6 +156,8 @@ class V2Controller(object):
     def _lookup(self, kind, *remainder):
         if (kind in ['meters', 'resources', 'samples']
                 and self.gnocchi_is_enabled):
+            if kind == 'meters' and pecan.request.method == 'POST':
+                return meters.MetersController(), remainder
             gnocchi_abort()
         elif kind == 'meters':
             return meters.MetersController(), remainder

ceilometer/tests/functional/api/v2/test_api_upgrade.py

@@ -65,6 +65,14 @@ class TestAPIUpgradePath(v2.FunctionalTest):
                                       "limit": 3
                                   }, status=410)
         self.assertIn(b'Gnocchi API', response.body)
+        self.post_json('/meters/fake_counter',
+                       params=[{
+                           "counter_type": "gauge",
+                           "counter_name": "fake_counter",
+                           "resource_id": "fake_resource_id",
+                           "counter_unit": "fake_unit",
+                           "counter_volume": "1"
+                       }], status=201)
 
     def _do_test_alarm_redirect(self):
         response = self.app.get(self.PATH_PREFIX + '/alarms',