Use six.StringIO

In links: http://pythonhosted.org/six/, we know 'cStringIO' is
a module of python 2.x, but in python3.x module 'io' has the
same function as 'cStringIO' in python2.x. So here we use
six.stringIO instead.

TrivialFix.

Change-Id: I8022b92b186e239cd76a36dab7766301bf96d2c4

ansible/action_plugins/merge_configs.py

@@ -15,9 +15,9 @@
 # limitations under the License.
 
 from ConfigParser import ConfigParser
-from cStringIO import StringIO
 import inspect
 import os
+from six import StringIO
 
 from ansible.plugins.action import ActionBase