Comment 2 for bug 1515402

The API itself does not support query by user name.
But The CLI support this.
  keystone user-get

As a work around you should query the user list at first,
for getting user-id user-name relation ship.
  http://developer.openstack.org/api-ref-identity-admin-v2.html#admin-listUsers

For a reference
  keystone cli command converts from name to id internally.
 you can see
    keystoneclient/v2_0/shell.py user-get(user_get)
    keystoneclient/v2_0/users.py query user info
    keystoneclient/base.py name to id conversion