rkt driver should remove pods when stopping tasks

[schmichael]

Nomad version

0.7.0

Operating system and Environment details

Linux, rkt 1.29.0

Issue

Docker removes containers once they exit: https://github.com/hashicorp/nomad/blob/master/client/driver/docker.go#L1685-L1688

rkt does not:

https://github.com/hashicorp/nomad/blob/master/client/driver/rkt.go#L769-L787

Reproduction steps

nomad run rkt.nomad # see below
nomad stop echo
rkt list
# echo's container still exists

Job file (if appropriate)

job "echo" {
  datacenters = ["dc1"]

  group "echo" {
    task "echo" {
      driver = "rkt"

      config {
        image = "docker://ubuntu"
        command = "/foo/ping"
        args    = ["google.com"]
        dns_servers = ["host"]
        volumes = ["/bin:/foo"]
      }

      resources {
        network {
          mbits = 10
        }
      }
    }
  }
}

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