qa/rgw: fix invalid syntax error in radosgw_admin_rest.py

[cbodley]

Fixes: http://tracker.ceph.com/issues/37440

[cbodley]

passes the new tests for user listing:

2018-11-30T17:57:39.977 INFO:tasks.radosgw_admin_rest:radosgw-admin-rest: ['user', 'list'] {'list': '', 'max-entries': 0}
2018-11-30T17:57:39.986 INFO:tasks.radosgw_admin_rest: json result: {u'keys': [], u'count': 0, u'marker': u'6:1e6ca5ed:users.uid::foo:head', u'truncated': True}
2018-11-30T17:57:39.986 INFO:tasks.radosgw_admin_rest:radosgw-admin-rest: ['user', 'list'] {'list': '', 'max-entries': 1}
2018-11-30T17:57:39.995 INFO:tasks.radosgw_admin_rest: json result: {u'keys': [u'foo'], u'count': 1, u'marker': u'6:e1d59a8c:users.uid::ada:head', u'truncated': True}
2018-11-30T17:57:39.995 INFO:tasks.radosgw_admin_rest:radosgw-admin-rest: ['user', 'list'] {'marker': u'6:e1d59a8c:users.uid::ada:head', 'list': '', 'max-entries': 1}
2018-11-30T17:57:40.000 INFO:tasks.radosgw_admin_rest: json result: {u'keys': [u'ada'], u'count': 1, u'truncated': False}

http://pulpito.ceph.com/cbodley-2018-11-30_17:39:44-rgw-wip-cbodley-testing-distro-basic-smithi/

[cbodley]

thanks for review!