Test KeyCDN Zero Downtime migration steps

[gweiying]

To buy custom domain cert and test KeyCDN Zero Downtime migration steps link.

Background

In our current steps for switching from custom certs to letsencrypt on KeyCDN, we experience a downtime between 10-20 mins during which KeyCDN tries to acquire letsencrypt certs. However, they detail a method for zero-downtime switches above. We plan to buy a custom domain cert and test out this method, so that future downtime between switches can reduced or negated.