pd: 😄 add a CLI flag to use the LetsEncrypt staging environment

[cratelyn]

in #3652, @conorsch suggested:

It'd be nice to expose this as a CLI opt (preserving default=true) so that in test-only scenarios, we can use the staging api. For now, when we need to do that, we've just been editing this source, which is a fine workaround for devs.

we should do this! it will likely prove to be useful with future testing of the auto-https code, and other integration testing. (see #3336, for other notes on testing)