Use base.get_host_for_server to get server host

This is to use base.get_host_for_server to get server
host in testcases.

Change-Id: I3a0c02778c07f3ec68fa22775dd59db196608a22
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index cdc30b9..dff50a9 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -704,6 +704,11 @@
         else:
             raise lib_exc.InvalidConfiguration()
 
+    @classmethod
+    def get_host_for_server(cls, server_id):
+        server_details = cls.os_admin.servers_client.show_server(server_id)
+        return server_details['server']['OS-EXT-SRV-ATTR:host']
+
 
 class NetworkScenarioTest(ScenarioTest):
     """Base class for network scenario tests.