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
Merge "Allow disabling or delaying the token_flush cron"