Allow using existing ACME certificates

Add a certificate scheme for using an existing ACME certificate without
setting up Nginx.

Also use names instead of magic numbers for certificate schemes.

docs/setup-guide.rst

@@ -81,7 +81,7 @@ these should be the most common ones.
 
        # Use Let's Encrypt certificates. Note that this needs to set up a stripped
        # down nginx and opens port 80.
-       certificateScheme = 3;
+       certificateScheme = "acme-nginx";
      };
    }