INGEST: Allow Repeated Invocation of Pipeline

[original-brownbear]

• Allows repeated, non-recursive invocation
  of the same pipeline

---

Without this change you can't run the following pipeline:

PUT _ingest/pipeline/outer-pipeline
{
  "description" : "outer pipeline",
  "processors" : [
    {
      "pipeline" : {
        "pipeline": "inner-pipeline"
      }
    },
    {
      "pipeline" : {
        "pipeline": "inner-pipeline"
      }
    }
  ]
}

because it will complain about inner-pipeline being executed twice (it incorrectly complains about "recursive" invocation here).
IMO, this should not throw as there may be valid use cases for using the same nested pipeline repeatedly to dry things up.

[elasticmachine]

Pinging @elastic/es-core-infra

[rjernst]

This seems likely to be a mistake on a users part, but it certainly shouldn't show up as a recursion. LGTM.

[original-brownbear]

@rjernst thanks!