Flush expired tokens from Keystone

Outdated tokens will be cleaned (removed) to avoid running out
of space in long term.

DocImpact
Change-Id: Ic688cc9cc03a4167f9cb55af799b326654e9ef13
Implements: blueprint access-control-master-node-improvments
(cherry picked from commit 0214e23d1ed58d1ae21550f9f7f710f3fb44557d)
Merge "Flush expired tokens from Keystone" into stable/5.1