certbot's own -d already takes a comma-separated list and puts every name
in the certificate's Subject Alternative Names, so `-d "$CERTBOT_DOMAIN"`
needed no change at all. What assumed a single domain was everything that
treated the variable as a *file name*: --cert-name, and the two scripts
that address the lineage under /etc/letsencrypt/live. Certbot names a
lineage after the first domain of the list, so all three now take
${CERTBOT_DOMAIN%%,*} instead of the whole string -- otherwise a two-domain
value asks for a lineage literally called "a.org,b.org" and the renewal
pass looks for a directory nobody made.
One certificate with several names rather than several lineages: HAProxy
binds a single site.pem, and a second lineage would have nowhere to go.
Adding a domain to CERTBOT_DOMAIN on a machine that already holds a
certificate still needs one manual issuance -- `certbot renew` reads the
names off the certificate it has and never looks at the variable. The
README says so, and gives the command.
