Instead of renewing certificates based on whether or not they're expired
at a fixed 14-day period in the future, renew based on whether or not
we're more than 2/3 of the way through the certificate's lifetime. This
properly handles shorter-lived certificates without issue.
Updates #8204
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I5e82a9cadc427c010d04ce58c7f932e80dd571ea