Enhance test_statistics
Enhance test_statistics

Fixed problems with test_list_statistics_with_no_merge_metrics
Removed test_list_statistics_with_merge_metrics
Removed test_list_statistics_with_more_than_one_statistics

Change-Id: I8283b67ba8de3d64e8cce636f3e87088b081ca40

openstack/monasca-api,sapcc/monasca-api,openstack/monasca-api,stackforge/monasca-api,sapcc/monasca-api,stackforge/monasca-api,stackforge/monasca-api,openstack/monasca-api,stackforge/monasca-api,sapcc/monasca-api