This PR implements support for check_restart for checks registered
in the Nomad service provider.
Unlike Consul, Nomad service checks never report a "warning" status,
and so the check_restart.ignore_warnings configuration is not valid
for Nomad service checks.
This PR introduces the `address` field in the `service` block so that Nomad
or Consul services can be registered with a custom `.Address.` to advertise.
The address can be an IP address or domain name. If the `address` field is
set, the `service.address_mode` must be set in `auto` mode.
