Deal with missing hosts in the host_to_roles_map more gracefully

[varilink]

If you inadvertently omit a host from the host_to_roles_map variable then that will cause playbook failure; for example when templating out the bacula-dir.conf file. The failures are easy enough diagnose but it would be better still if we executed a task right at the start of playbooks that validated the host_to_roles_map and gave a nice, clear "You're missing entries" message instead.