Fixes#8000
When requesting a Service Identity token from Consul, use the TaskKind
of the Task to get at the service name associated with the task. In
the past using the TaskName worked because it was generated as a sidecar
task with a name that included the service. In the Native context, we
need to get at the service name in a more correct way, i.e. using the
TaskKind which is defined to include the service name.
Nomad jobs may be configured with a TaskGroup which contains a Service
definition that is Consul Connect enabled. These service definitions end
up establishing a Consul Connect Proxy Task (e.g. envoy, by default). In
the case where Consul ACLs are enabled, a Service Identity token is required
for these tasks to run & connect, etc. This changeset enables the Nomad Server
to recieve RPC requests for the derivation of SI tokens on behalf of instances
of Consul Connect using Tasks. Those tokens are then relayed back to the
requesting Client, which then injects the tokens in the secrets directory of
the Task.
