nOutput_ES: provide field indexes to easier use in ES

[nmaguiar]

Each attribute value is sent to ES like:

{
   "name_of_attribute": {
      "key1": "value1",
      "property2": "value2",
      "property3": "value3"
   },
   "stamp_key": "value_key"
}

but to filter by key1, that might be used in other attributes, several specific filters might be needed. If an execArg is added to provide a list of key field indexes (in this case [ "key1" ]) the resulting value sent to ES would be:

{
   "name_of_attribute": {
      "property2": "value2",
      "property3": "value3"
   },
   "key1": "value1",
   "stamp_key": "value_key"
}