sentinel-proxy: Spread cert checks/renewals across 12-hour period

The current configuration checks Sentinel certificates (and renews them
eventually) twice a day at exact times. It leads to sharp load peeks on
the ca server and other parts of the Sentinel cert infrastructure. This
patch utilizes RANDOM_DELAY in the cron configuration to spread the load
across the whole 12-hour period. This delay (0 - 719 minutes) is
determined when the cron daemon starts and is kept during its operation
(thus it can be predicted when the check occurs).

collect/sentinel/sentinel-proxy/files/renew.cron

 ## crontab
 #
 
+# Random delay to spread checks/renewals across whole periods
+RANDOM_DELAY=719
+
 # Periodically check and renew Sentinel:Proxy certificate
-42 */12 * * * root sentinel-certgen certs --hooks-dir /usr/libexec/sentinel/renew_hooks.d
+0 */12 * * * root sentinel-certgen certs --hooks-dir /usr/libexec/sentinel/renew_hooks.d