Updated openstack/openstack
Project: openstack/ceilometer  90bae4a21d8f53b8843a5eeb8ccbba0f7e5e6b71

Fix docs to suit merged compute/central agents concept

DocImpact: Fix agents-related docs to describe polling agent
Related-Blueprint: merge-compute-central-agents

Change-Id: If377ac49bf75c0698f4af6be021e8354240f4e42