add a new sub command "stat" to display cinder statistics

currently we don't have any commands to get the statistics of cinder service.

If we have more number of volumes this command will help a lot to get the higher level information/statistics about cinder.

below is the proposed command detail

NAME - 'stat' or 'statistics'
PURPOSE - it will display the current statistics about cinder.

admin user can pass --all-tenant attribute to get the statistics for all the tenants.

SAMPLE OUTPUT:

devstack$ cinder stat
+----------------------------------+-------+
| | Value |
+----------------------------------+-------+
| Total number of Volumes | 2 |
| Total size of All volumes | 3 |
| Total number of Attached volumes | 0 |
| Total number of Free volumes | 2 |
| Total number of Bottable volumes | 0 |
+----------------------------------+-------+