This PR updates Nomad's Consul service client to include checks in
an initial service registration, so that the checks associated with
the service are registered "atomically" with the service. Before, we
would only register the checks after the service registration, which
causes problems where the service is deemed healthy, even if one or
more checks are unhealthy - especially problematic in the case where
SuccessBeforePassing is configured.

Fixes #3935