r/ssh-host-certs: Manage SSH host certificates

The *ssh-host-certs* role, which is now applied as part of the
`base.yml` playbook and therefore applies to all managed nodes, is
responsible for installing the *sshca-cli* package and using it to
request signed SSH host certificates.  The *sshca-cli-systemd*
sub-package includes systemd units that automate the process of
requesting and renewing host certificates.  These units need to be
enabled and provided the URL of the SSHCA service.  Additionally, the
SSH daemon needs to be configured to load the host certificates.

group_vars/all.yml

@@ -1,3 +1,5 @@
+sshca_url: https://sshca.pyrocufflink.blue
+
 certbot_account_email: dustin@hatch.name
 smtp:
   mode: relay