Add decorator to skip test in case when DVR router is used

Some tests are valid only for centralized routers, so it
adds a new decorator method skip_if_router_distributed to skip
tests when router used in the NetworkStackFixture is DVR.

It uses the new decorator in below test case:
 tobiko/tests/faults/neutron/test_ovs.py::OpenvswitchTest::test_recreate_physical_bridge

Change-Id: I46128d5f76eef7428b765232c705b7b1a6b760d5

tobiko/openstack/stacks/_neutron.py

@@ -291,6 +291,21 @@ class NetworkStackFixture(heat.HeatStackFixture):
             requirements['router'] += 1
         return requirements
 
+    def is_router_distributed(self) -> bool:
+        if self.has_gateway:
+            tobiko.setup_fixture(self)
+            return bool(self.gateway_details.get('distributed'))
+        else:
+            return False
+
+    @classmethod
+    def skip_if_router_is_distributed(cls, reason: str = None):
+        fixture = tobiko.get_fixture(cls)
+        if reason is None:
+            reason = "Distributed router is not supported"
+        return tobiko.skip_if(reason=reason,
+                              predicate=fixture.is_router_distributed)
+
 
 @neutron.skip_if_missing_networking_extensions('net-mtu-writable')
 class NetworkWithNetMtuWriteStackFixture(NetworkStackFixture):

tobiko/tests/faults/neutron/test_ovs.py

@@ -31,7 +31,8 @@ from tobiko.tripleo import undercloud
 @neutron.skip_if_missing_networking_agents(neutron.L3_AGENT)
 class OpenvswitchTest(testtools.TestCase):
 
-    stack = tobiko.required_setup_fixture(stacks.CirrosServerStackFixture)
+    stack: stacks.ServerStackFixture = tobiko.required_fixture(
+        stacks.CirrosServerStackFixture)
 
     def setUp(self):
         super(OpenvswitchTest, self).setUp()
@@ -72,6 +73,7 @@ class OpenvswitchTest(testtools.TestCase):
         raise neutron.AgentNotFoundOnHost(agent_type="neutron-ovs-agent",
                                           host=hostname)
 
+    @stacks.NetworkStackFixture.skip_if_router_is_distributed()
     @undercloud.skip_if_missing_undercloud
     def test_recreate_physical_bridge(self):
         # Check if vm is reachable before test