This skips several tasks that fail, at least in some conditions, when
run in check mode but not when run in normal mode.

As I noted in an earlier commit, restricting Docker's ingress traffic is
more complicated than adding a few rules to the filter table's INPUT
chain. I had thought that relying on LOCKSS's "LOCKSS_ACCESS_SUBNET"
variable would be sufficient; unfortunately, that is not the case:
LOCKSS sees all of its traffic as coming from the Docker overlay network
(by default, 10.0.0.0/8), regardless of its true origin.

Fortunately for us, Docker provides us with a chain, DOCKER-USER, called
by Docker's rules from the FORWARD chain in the filter table, that is
suitable for filtering Docker's ingress traffic.

Accordingly, this commit:

- Removes the misleadingly ineffective 'lockss_trusted_ips' variable and
  provides new variables 'lockss_network_ips' and 'lockss_admin_ips'.

- Replaces ufw or firewalld with ferm. Neither ufw nor firewalld is
  capable of satisfying this use case without employing significant
  violence; ferm is elegant and has beautiful configuration files.
  Beauty over bloodshed.

- Adds tasks to configure a local firewall that denies inbound and
  forwarded traffic by default, permits ssh from anywhere, and permits
  access to LOCKSS's data and configuration ports from
  'lockss_network_ips' and 'lockss_admin_ips' respectively.

  Said firewall cooperates with Docker: Docker and the firewall can be
  started in any order, and restarting either preserves rules created by
  the other.

                                *  *  *

I got the suggestion of using ferm for this, as well as the bulk of the
Docker-related rules, from a blog post: Ben Chavet, Convincing Docker
and Iptables to Play Nicely, Aug. 8, 2019,
<https://www.lullabot.com/articles/convincing-docker-and-iptables-play-nicely>.