Fix misleading docstring
Fix misleading docstring

In I3683fce981a7d1812d08f3545e9a8b1d8ebb6ec1 we incorrectly indentified
the necessary key `user` as `username`.

Change-Id: Ib6e90a9187024d7ffd02a8f011d7ad0f759ef733

openstack/python-saharaclient