Core: The `index.query.parse.strict` setting also fails if queries refer to unmapped fields.

Also the percolator and filter parsing for aliases enforce strict query parsing

Strict parsing for percolator and filter alias parsing is only enforced on indices created after to upgrade to `1.4.0`

PR for #6664
Can you make it a constant so that tests can use it?

Also, maybe it should be `index.percolator.allow_unmapped_fields` instead? (removing the double negation when it is set to `false` and using underscores for the last part, what do you think?)