puppet: Prevent unattended upgrades of erlang-base.

When upgraded, the `erlang-base` package automatically stops all
services which depend on the Erlang runtime; for Zulip, this is the
`rabbitmq-server` service.  This results in an unexpected outage of
Zulip.

Block unattended upgrades of the `erlang-base` package.

puppet/zulip_ops/files/apt/apt.conf.d/50unattended-upgrades

@@ -22,6 +22,7 @@ Unattended-Upgrade::Package-Blacklist {
     "nginx-full$";
     "postgresql-\d+$";
     "rabbitmq-server$";
+    "erlang-base$";
     "redis-server$";
     "supervisor$";
 };