logstash: Update default mappings for Elasticsearch 2.x

Update the default mapping template to coerce all non-string fields
without explicit mappings to strings.

Starting with Elasticsearch 2.x, fields with the same name, in the same
index, in different types, must have the same mapping. This is
problematic for our Logstash traffic where different applications may
(and do!) use common names like "code" and "status" as structured log
data with differing content. We have a "normalize_fields" filter that
has been used to try and clean up these differences, but that is
a fragile approach that could be broken at any time by a new application
or new event type for an existing application that has conflicts with
anything else in our logging environment.

Bug: T136001
Change-Id: I638d88e1d874fdb8be211bd74a1e36998d42dc09