Ceilometer: "Error: Unable to retrieve tenant list." on /admin/metering/

[Yves-Gwenael Bourhis]

Steps to reproduce:
================
* Configure ceilometer in your devstack's localrc:

# Ceilometer
CEILOMETER_BACKEND=mongodb
# Enable the ceilometer metering services
enable_service ceilometer-acompute,ceilometer-acentral,ceilometer-collector
# Enable the ceilometer alarming services
enable_service ceilometer-alarm-evaluator,ceilometer-alarm-notifier
# Enable the ceilometer api services
enable_service ceilometer-api

configure keystone API 2.0 in openstack_dashboard/local/local_settings.py :
OPENSTACK_API_VERSIONS = {
    "identity": 2.0
}

* run your devstack
* run horizon (if it's noot running on your devstack)
* navigate to /admin/metering/ (Resource Usage) in you browser.

Bug Detail:
=========

In the browser you see the error message "Error: Unable to retrieve tenant list."

In horizon's logs we see:
REQ: curl -i -X GET http://192.168.56.4:35357/v2.0/tenants?marker=tenant_marker&limit=21 -H "User-Agent: python-keystoneclient" -H "Forwarded: for=127.0.0.1;by=python-keystoneclient" -H "X-Auth-Token: d53d0e0b07101c75b76d9528fab50fc1"
RESP: [400] CaseInsensitiveDict({'date': 'Tue, 19 Nov 2013 09:40:23 GMT', 'vary': 'X-Auth-Token', 'content-length': '88', 'content-type': 'application/json'})
RESP BODY: {"error": {"message": "Marker could not be found", "code": 400, "title": "Bad Request"}}

Request returned failure status: 400
WARNING 2013-11-19 09:40:23,416 /home/yves/openstack/horizon/horizon/exceptions.py:310 handle Recoverable error: Marker could not be found (HTTP 400)

removing 'marker="tenant_marker"' here:
openstack_dashboard/dashboards/admin/metering/tabs.py:34: marker="tenant_marker")
openstack_dashboard/dashboards/admin/metering/views.py:149: marker="tenant_marker")
makes the "Resource usage" page work for me.