Added negative tests for server

Here two negative tests are added:-
 *Pausing a non-existent server
 *Stoping a non existent server

Change-Id: Iac9642d17907e1628914a028ea32bebfb40624f6
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index 703f143..e09a23f 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -255,6 +255,20 @@
         self.assertRaises(exceptions.NotFound, self.client.get_server,
                           '999erra43')
 
+    @attr(type=['negative', 'gate'])
+    def test_stop_non_existent_server(self):
+        # Stop a non existent server
+        non_exist_id = rand_name('non-existent-server')
+        self.assertRaises(exceptions.NotFound, self.servers_client.stop,
+                          non_exist_id)
+
+    @attr(type=['negative', 'gate'])
+    def test_pause_non_existent_server(self):
+        # pause a non existent server
+        non_exist_id = rand_name('non-existent-server')
+        self.assertRaises(exceptions.NotFound, self.client.pause_server,
+                          non_exist_id)
+
 
 class ServersNegativeTestXML(ServersNegativeTestJSON):
     _interface = 'xml'