This commit adds the ability to require an ingest pipeline on an
index. Today we can have a default pipeline, but that could be
overridden by a request pipeline parameter. This commit introduces a new
index setting index.required_pipeline that acts similarly to
index.default_pipeline, except that it can not be overridden by a
request pipeline parameter. Additionally, a default pipeline and a
request pipeline can not both be set. The required pipeline can be set
to _none to ensure that no pipeline ever runs for index requests on that
index.