Fix check of the revision number in the resonse from API

In the test_router_interface_fip test module there was probably some
copy paste error which was checking if revision_number key exists in the
older response from the API then what was used in the assertion.
Now this is fixed.

TrivialFix

Change-Id: Ica10dfec9bb806c3dca2f0a2e0aff36006c9c092
diff --git a/neutron_tempest_plugin/api/test_router_interface_fip.py b/neutron_tempest_plugin/api/test_router_interface_fip.py
index 5d8ab67..dcaa17c 100644
--- a/neutron_tempest_plugin/api/test_router_interface_fip.py
+++ b/neutron_tempest_plugin/api/test_router_interface_fip.py
@@ -83,7 +83,7 @@
                                fip2['revision_number'])
         # NOTE(yamamoto): The status can be updated asynchronously.
         fip2_shown = self.client.show_floatingip(fip2['id'])['floatingip']
-        if 'revision_number' in fip2:
+        if 'revision_number' in fip2_shown:
             self.assertGreaterEqual(fip2_shown['revision_number'],
                                     fip2_updated['revision_number'])
         fip2_shown.pop('status')