Ensure that we run token-flush cron job under correct user.

Also remove creation of /var/log/keystone/keystone.log as it should be created correctly now.
Closes-bug: rhbz#1071674

Change-Id: I450d89d47527db101f56bf173062ba33ceb3b54d
(cherry picked from commit 9b7eb96e937d2a05bd216d82c6043095642524b1)