refactor: don't require host= for check_connectivity

When ssh_client is passed, the argument is not used.

Change-Id: Ibf73130fbf82c2ed85e16b3f69aacbc2930efb3d
diff --git a/neutron_tempest_plugin/scenario/base.py b/neutron_tempest_plugin/scenario/base.py
index 6149b06..70cb2dc 100644
--- a/neutron_tempest_plugin/scenario/base.py
+++ b/neutron_tempest_plugin/scenario/base.py
@@ -337,7 +337,7 @@
         if create_fip:
             self.fip = self.create_floatingip(port=self.port)
 
-    def check_connectivity(self, host, ssh_user=None, ssh_key=None,
+    def check_connectivity(self, host=None, ssh_user=None, ssh_key=None,
                            servers=None, ssh_timeout=None, ssh_client=None):
         # Either ssh_client or ssh_user+ssh_key is mandatory.
         if ssh_client is None: