add SSL/TLS support to Alpine MQTT infrastructure
We are currently only supporting plain text MQTT and MQTT+Websockets on
msg.alpinelinux.org.
This should be extended with MQTT over SSL/TLS and Websockets over
SSL/TLS.
To migrate all infra to SSL/TLS mqtt-exec will need to be updated with
SSL/TLS support.
See: https://github.com/ncopa/mqtt-exec/issues/4
(from redmine: issue id 8475, created on 2018-02-05)