Use StringIO from io

We don't test CfnInitIntegrationTest.test_server_cfn_init and hence
this was not detected.

Change-Id: I7bc1fe17e8d9149c93e838dfe4a253a69387c480
diff --git a/heat_tempest_plugin/common/remote_client.py b/heat_tempest_plugin/common/remote_client.py
index aa652be..a7b9ab7 100644
--- a/heat_tempest_plugin/common/remote_client.py
+++ b/heat_tempest_plugin/common/remote_client.py
@@ -33,7 +33,7 @@
         self.password = password
         if isinstance(pkey, str):
             pkey = paramiko.RSAKey.from_private_key(
-                io.cStringIO(str(pkey)))
+                io.StringIO(str(pkey)))
         self.pkey = pkey
         self.look_for_keys = look_for_keys
         self.key_filename = key_filename