Allow disabling or delaying the token_flush cron

Add the max_delay parameter to prevent running all cron jobs at the
exact same time in case this cron job is installed on more than one
server. The default is to not wait though.

Add the ensure parameter to allow removing the cron job.

Change-Id: I1cd231305a1ecc642c638af6cef7d591d1e3fe3e
(cherry picked from commit 8e502087aa2c3a4970fac7a33622072a377e9bdf)