[Nomad] nomad-jobspec not work with node_pool

[dominhhai]

Describe the bug
The generated JobSpecs do not include the node_pool because of old nomad' api version which ref from Waypoint builtin.

Steps to Reproduce
Write node_pool to app.nomad.tpl

job "web" {
  datacenters = ["dc1"]
  node_pool   = "dev"
  group "app" {
    update {
      max_parallel = 1
      canary       = 1
      auto_revert  = true
      auto_promote = false
      health_check = "task_states"
    }

    task "app" {
      driver = "docker"
      config {
        image = "${artifact.image}:${artifact.tag}"
      }

      env {
        // For URL service
        PORT = "3000"
      }
    }
  }
}

Then, the generated JobSpecs as

{
   ### before
   "NodePool": "default",
   ### after
}

Expected behavior
The node_pool in the job template will be included in the generated JobSpecs.

Waypoint Platform Versions

• Waypoint CLI Version: v0.11.4 (7128fba)
• Waypoint Server Platform and Version: nomad v0.11.4
• Waypoint Plugin: nomad jobspec
• Nomad Version: v1.6.1