ACME: use HTTP for the Nginx readyness check

[dpausp]

This avoids conflicts with other services that want to use port 443.
Our current use case for this is a TURN server using 443.
Nginx can be configured to only listen on port 80 now.
The acme client uses HTTP for challenges.

Case 126629

@flyingcircusio/release-managers

Release process

Impact:

Changelog:

• Fix port conflict on 443 when Letsencrypt is used. This makes it possible to run a TURN server on 443 with automated letsencrypt certs, for example.

Security implications

• Security requirements defined? (WHERE)
  • only affects an internal check on a port that must be open anyways for letsencrypt
• Security requirements tested? (EVIDENCE)
  - nothing to test