Run SNAT specific test cases only with ext-gw-mode extension

In network api's test_routers, there are test cases which involves
enable_snat attribute. We should not be running these test cases if
the ext-gw-mode extension is not supported by network plugin.
Adding decorator to corresponding methods in test_routers.

Change-Id: I71fd96ce6b30ba7307c92733c363eb996ee6c1db
Closes-Bug: #1273360
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index 426273c..7cabb63 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -149,6 +149,7 @@
             {'network_id': self.network_cfg.public_network_id})
         self._verify_gateway_port(router['id'])
 
+    @test.requires_ext(extension='ext-gw-mode', service='network')
     @test.attr(type='smoke')
     def test_update_router_set_gateway_with_snat_explicit(self):
         router = self.create_router(data_utils.rand_name('router-'))
@@ -163,6 +164,7 @@
              'enable_snat': True})
         self._verify_gateway_port(router['id'])
 
+    @test.requires_ext(extension='ext-gw-mode', service='network')
     @test.attr(type='smoke')
     def test_update_router_set_gateway_without_snat(self):
         router = self.create_router(data_utils.rand_name('router-'))
@@ -190,6 +192,7 @@
             device_id=router['id'])
         self.assertFalse(list_body['ports'])
 
+    @test.requires_ext(extension='ext-gw-mode', service='network')
     @test.attr(type='smoke')
     def test_update_router_reset_gateway_without_snat(self):
         router = self.create_router(