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 adds support for specifying checks in services registered to
the built-in nomad service provider.
Currently only HTTP and TCP checks are supported, though more types
could be added later.