Modify some help and error messages in ec2creds identityv2

Usually we use "(s)" to show about multi deletion in
help message. In addition, I think "EC2 credentials"
is better than "EC2 keys" in the error message.

Change-Id: I6a6461291542701d87a55d9ea0ea1fda6db04601

Author: Huanxuan Ao

openstackclient/identity/v2_0/ec2creds.py

@@ -94,7 +94,7 @@ def get_parser(self, prog_name):
             'access_keys',
             metavar='<access-key>',
             nargs='+',
-            help=_('Credentials access keys'),
+            help=_('Credentials access key(s)'),
         )
         parser.add_argument(
             '--user',
@@ -121,7 +121,7 @@ def take_action(self, parsed_args):
                 identity_client.ec2.delete(user, access_key)
             except Exception as e:
                 result += 1
-                LOG.error(_("Failed to delete EC2 keys with "
+                LOG.error(_("Failed to delete EC2 credentials with "
                           "access key '%(access_key)s': %(e)s")
                           % {'access_key': access_key, 'e': e})