Some CentOS 7 hosts may not have firewalld installed.
We shouldn't expect it to be, but should ensure by ourselves that it is.

Docker likes to mess around with iptables forwarding rules,
so it ought to start after firewalld.

Doing this means that matrix-nginx-proxy now starts
occupying port 80, which necessitates that
SSL renewal happens slightly differently.

matrix-nginx-proxy will be occupying port 80 soon,
so that we can be more user-friendly and have
http->https forwarding for the Riot hostname.

During the playbook run, acmetool also expects to use
port 80 for domain verification.

During an initial playbook run, this wouldn't cause trouble
because matrix-nginx-proxy is not installed yet.

However, on subsequent playbook runs, it would cause trouble.

This ensures that if matrix-nginx-proxy is available
and running, it would be stopped before running acmetool
and started right after.