Require l3-ha extension for test_centralized_router_update_to_dvr

Since Ib262f32133aae50cdb01d2489830499fb5c30f44, the test explicitly
passes ha=False argument when creating a centralized router. This
attribute belongs to l3-ha API extension, and may be not present for
some neutron setups (f.e. when using networking-odl).

Skip the test if the extension is not enabled.

Change-Id: I71059bb2fd8c13270b7a9c3f0a6a07de310b26fc
diff --git a/tempest/api/network/admin/test_routers_dvr.py b/tempest/api/network/admin/test_routers_dvr.py
index 7de3760..66feba8 100644
--- a/tempest/api/network/admin/test_routers_dvr.py
+++ b/tempest/api/network/admin/test_routers_dvr.py
@@ -13,6 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import testtools
+
 from tempest.api.network import base_routers as base
 from tempest.common.utils import data_utils
 from tempest import test
@@ -80,6 +82,8 @@
         self.assertFalse(router['router']['distributed'])
 
     @test.idempotent_id('acd43596-c1fb-439d-ada8-31ad48ae3c2e')
+    @testtools.skipUnless(test.is_extension_enabled('l3-ha', 'network'),
+                          'HA routers are not available.')
     def test_centralized_router_update_to_dvr(self):
         """Test centralized router update