Reuse mocked body value between tests

Unit tests of service clients exist for both string/bytes bodies
and these bodies were defined as different value.
However, they are different only for type and current definitions
are redundant. So this patch makes one value reused for code cleanup.

Change-Id: I80dd14adc52b865ad53b55772fe8989dc7df55ec
diff --git a/tempest/tests/services/compute/test_agents_client.py b/tempest/tests/services/compute/test_agents_client.py
index e8ea525..d268a18 100644
--- a/tempest/tests/services/compute/test_agents_client.py
+++ b/tempest/tests/services/compute/test_agents_client.py
@@ -32,10 +32,9 @@
                                                  'compute', 'regionOne')
 
     def _test_list_agents(self, bytes_body=False):
+        body = '{"agents": []}'
         if bytes_body:
-            body = bytes(b'{"agents": []}')
-        else:
-            body = '{"agents": []}'
+            body = bytes(body.encode('utf-8'))
         expected = []
         response = (httplib2.Response({'status': 200}), body)
         self.useFixture(mockpatch.Patch(