Automatic merge from submit-queue (batch tested with PRs 44364, 44361, 42498)

Fix the certificate rotation threshold and add jitter.

Adjusts the certificate rotation threshold to be fixed, with some jitter to
spread out the load on the Certificate Signing Request API. The rotation
threshold is fixed at 20% now, meaning when 20% of the certificate's total
duration is remaining, the certificate manager will attempt to rotate, with
jitter +/-10%. For certificates of duration 1 month that means they will
rotate after 24 days, +/- 3 days.

On a 6000 node cluster, assuming all nodes added at nearly the same time, this
should result in 6000 nodes rotating spread over 6 days (total range of the
jitter), or ~42 nodes / hour requesting new certificates.