Sort list images by name by default

Set the default key and dir for openstack image list,
if no command line argument passed
Changing sort_key to name(old value: created_at)

Change-Id: I6c61f6e5a04824d655ccf43477afcec9652101df
Closes-Bug: #1639231

doc/source/command-objects/image.rst

@@ -221,7 +221,7 @@ List available images
 
 .. option:: --sort <key>[:<direction>]
 
-    Sort output by selected keys and directions(asc or desc) (default: asc),
+    Sort output by selected keys and directions(asc or desc) (default: name:asc),
     multiple keys and directions can be specified separated by comma
 
 .. option:: --limit <limit>

openstackclient/image/v1/image.py

@@ -350,8 +350,9 @@ class ListImage(command.Lister):
         parser.add_argument(
             '--sort',
             metavar="<key>[:<direction>]",
+            default='name:asc',
             help=_("Sort output by selected keys and directions(asc or desc) "
-                   "(default: asc), multiple keys and directions can be "
+                   "(default: name:asc), multiple keys and directions can be "
                    "specified separated by comma"),
         )
         return parser

openstackclient/image/v2/image.py

@@ -447,8 +447,9 @@ class ListImage(command.Lister):
         parser.add_argument(
             '--sort',
             metavar="<key>[:<direction>]",
+            default='name:asc',
             help=_("Sort output by selected keys and directions(asc or desc) "
-                   "(default: asc), multiple keys and directions can be "
+                   "(default: name:asc), multiple keys and directions can be "
                    "specified separated by comma"),
         )
         parser.add_argument(

releasenotes/notes/bug-1639231-21823768bd54170a.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    The  ``image list`` command will now sort by name in ascending order by
+    default. ``--sort`` option will have the default value of ``name:asc``.
+    [Bug `1639231 <https://bugs.launchpad.net/bugs/1639231>`_]