This change introduces a new /stats REST endpoint that
provide the following basic information;
1) Total number of clusters and nodes for the given tenant.
2) Total number of clusters and nodes across all the tenants.
Follow-up patches include more stats.
Change-Id: Iac0bf9343549de31654545d5b1fd7601e56142a7
Partially Implements blueprint magnum-stats-api
Vijendar Komalla