Add -d short option for --debug

A short option for --debug is a convenience.
This commit adds the -d short option as a shortcut for --debug

Glance already provides -d.

Change-Id: Iefae6cee566f9c7d7375e89c3f8603f149e555ef
Closes-Bug: #1421210

README.rst

@@ -110,7 +110,7 @@ You'll find complete documentation on the shell by running
         list-extensions     List all the os-api extensions that are available.
 
     Optional arguments:
-      --debug               Print debugging output
+      -d, --debug               Print debugging output
       --os-username <auth-user-name>
                             Defaults to env[OS_USERNAME].
       --os-password <auth-password>

cinderclient/shell.py

@@ -125,7 +125,7 @@ class OpenStackCinderShell(object):
                             action='version',
                             version=cinderclient.__version__)
 
-        parser.add_argument('--debug',
+        parser.add_argument('-d', '--debug',
                             action='store_true',
                             default=utils.env('CINDERCLIENT_DEBUG',
                                               default=False),