T6983: treat vyos-domain-resolver as a real service (#4261)

• op-mode: T6983: add "show log" and "monitor log" commands for vyos-domain-resolver

• firewall: T6983: drop unused variable domain_resolver_usage_nat

• T6983: introduce Python logging module in vyos-domain-resolver

• T6983: treat vyos-domain-resolver as a real service