Use pebble for all/more of our tests requiring an ACME server?

[bmw]

This isn't a high priority for us, but @jsha, when you get the time, I'd love to hear your recommendation here. The main benefit for us is pebbles are more lightweight than boulders.

cc @joohoi

[jsha]

I think that's probably a reasonable approach now, especially since ACME is all-but-done. When we were revving the ACME protocol support in Boulder frequently it made more sense to verify there was a close match between Boulder and Certbot, but now I think Pebble is sufficient (and testing is what it's for!).

[stale]

We've made a lot of changes to Certbot since this issue was opened. If you still have this issue with an up-to-date version of Certbot, can you please add a comment letting us know? This helps us to better see what issues are still affecting our users. If there is no further activity, this issue will be automatically closed.

[adferrand]

And now Pebble is used instead of Boulder during PRs integration tests. I think we can close this.