Fix tests updating server by name rather than id

Some tests in test_servers_negative.py update server by name rather
than id. According to the api spec[1], we should update it with id rather
name as this. PUT {tenant_id}/servers/{server_id}
[1]http://developer.openstack.org/api-ref-compute-v2.html#updateServer

Change-Id: I3b5f7831c334614e4824b99fba518a27bff3aa48
Closes-bug: #1489068
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index 5a14418..ae330e8 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -243,22 +243,21 @@
     def test_update_name_of_non_existent_server(self):
         # Update name of a non-existent server
 
-        server_name = data_utils.rand_name('server')
+        nonexistent_server = data_utils.rand_uuid()
         new_name = data_utils.rand_name('server') + '_updated'
 
         self.assertRaises(lib_exc.NotFound, self.client.update_server,
-                          server_name, name=new_name)
+                          nonexistent_server, name=new_name)
 
     @test.attr(type=['negative'])
     @test.idempotent_id('38204696-17c6-44da-9590-40f87fb5a899')
     def test_update_server_set_empty_name(self):
         # Update name of the server to an empty string
 
-        server_name = data_utils.rand_name('server')
         new_name = ''
 
         self.assertRaises(lib_exc.BadRequest, self.client.update_server,
-                          server_name, name=new_name)
+                          self.server_id, name=new_name)
 
     @test.attr(type=['negative'])
     @test.idempotent_id('543d84c1-dd2e-4c6d-8cb2-b9da0efaa384')