data/template: Do not index the arguments field (it's in _source)

As this field is randomly large depending on the task at end, this
triggers a limit (index.mapping.total_fields.limit to 1000). We do not
really need this in the index as the data will still be in the
_source.

[1] https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html#mapping-limit-settings

Related T1023