Validate default similarity parameters strictly

[eskibars]

Elasticsearch version (bin/elasticsearch --version): 6.2.0

JVM version (java -version): 1.8

OS version (uname -a if on a Unix-like system): OSX

Description of the problem including expected versus actual behavior:

Steps to reproduce:

PUT test
{
  "settings": {
    "index" : {
        "similarity" : {
          "default" : {
            "type" : "bm25"
          }
        }
    }
  }
}

Produces the expected: "Unknown Similarity type [bm25] for [default]" since type needs to be BM25. However (at least for BM25 and classic), it's happy to accept/acknowledge arbitrary parameters that may not exist:

PUT test
{
  "settings": {
    "index" : {
        "similarity" : {
          "default" : {
            "type" : "BM25",
            "foo": "bar"
          }
        }
    }
  }
}

Validates even though there's obviously no foo parameter. I had a situation where I had a typo of k instead of k1 and I was thrown off for a little too long trying to figure out why my k value wasn't taking hold. It'd be great if we could strictly validate and error out as early as possible.

[elasticmachine]

Pinging @elastic/es-core-infra

[elasticmachine]

Pinging @elastic/es-search-aggs