acme-common: Export canonical paths for storing certificates and challenges

acme-common: Export canonical paths for storing certificates and challenges

The contract between the acme-common framework and consumers and hook
scripts is that certificates can be consumed from /etc/ssl/acme and that
web challenges are stored in /var/run/acme/challenge. Make this explicit by
exporting $CERT_DIR and $CHALLENGE_DIR as environment variables as well,
instead of having knowledge of those paths depend on out-of-band
information. We already exported $challenge_dir, but let's change it to
upper-case to make it clear that it's not a user configuration variable.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>