Add command to start clustertasks similar to tasks

[siamaksade]

ClusterTask and Task should have the same commands exposed on CLI. T

$ tkn task
  ...

Available Commands:
  create      Create a task in a namespace
  delete      Delete a task resource in a namespace
  describe    Describes a task in a namespace
  list        Lists tasks in a namespace
  start       Start tasks

while

$ tkn clustertask
  ...

Available Commands:
  delete      Delete a clustertask resource in a cluster
  list        Lists clustertasks in a namespace

[vdemeester]

/kind feature

[danielhelfand]

/assign