Run keystone-manage token_flush cmd

Because the Identity server's token database table grows unconditionally
over time as new tokens are generated, the token table needs to be
manually cleared. The administrator must run the keystone-manage token_flush
command on a regular basis. Recommended frreqencyis once every minute.

Change-Id: Ie3dbe5e4336af71af7b83d68d28a1a72471f53eb
Closes-Bug: #1315236
Merge "Run keystone-manage token_flush cmd"