Add object storage collector for ceilometer

[David Höppner]

OpenStack object storage (Swift) driver is missing in the ceilometer collector. Add a driver for the storage.objects.size metric.

[David Höppner (0xffea)]

The metadata field is empty. Somebody know how to request the metrics
from the links attribute (with ceilometerclient)?

<Resource {u'user_id': None, u'links': [{u'href': u'http://127.0.0.1:8777/v2/resources/49df73c137f247529fc08bf2060085e6', u'rel': u'self'}, {u'href': u'http://127.0.0.1:8777/v2/meters/storage.objects.outgoing.bytes?q.field=resource_id&q.value=49df73c137f247529fc08bf2060085e6', u'rel': u'storage.objects.outgoing.bytes'}, {u'href': u'http://127.0.0.1:8777/v2/meters/storage.objects.containers?q.field=resource_id&q.value=49df73c137f247529fc08bf2060085e6', u'rel': u'storage.objects.containers'}, {u'href': u'http://127.0.0.1:8777/v2/meters/storage.objects?q.field=resource_id&q.value=49df73c137f247529fc08bf2060085e6', u'rel': u'storage.objects'}, {u'href': u'http://127.0.0.1:8777/v2/meters/storage.objects.size?q.field=resource_id&q.value=49df73c137f247529fc08bf2060085e6', u'rel': u'storage.objects.size'}], u'resource_id': u'49df73c137f247529fc08bf2060085e6', u'source': u'openstack', u'last_sample_timestamp': u'2016-10-04T12:44:13.418658', u'first_sample_timestamp': u'2016-10-04T12:24:12.852032', u'project_id': u'49df73c137f247529fc08bf2060085e6', u'metadata': {}}>