r/postgresql-cert: ACME certificates using certbot

This role can be used to get a server certificate for PostgreSQL from an
ACME CA using `certbot`.  It fetches the initial certificate and copies
it to the PostgreSQL configuration directory.  It also sets up a
post-renewal hook script that copies updated certificates and reload
the server.

roles/postgresql-cert/templates/certbot-renew.timer.j2

@@ -0,0 +1,3 @@
+[Timer]
+RandomizedDelaySec=15m
+OnCalendar=hourly