Add validation to privilege names in roles API

[elasticmachine]

Original comment by @ppf2:

I have a feeling that we may already have this on the roadmap and it's just not in the first iteration of the API. Filing to track progress since this will likely come up in the future. Currently, there is no validation on the values of the privileges set when creating roles. So it's possible for users to have a typo on the privilege names and the role will still get added/accepted by the API.

{
  "indices": [
    {
      "names": [ "test",".kibana" ],
      "privileges": [ "al" ]
    }
  ]
}

{
  "role": {
    "created": true
  }
}

It will be nice if we can validate this and throw back and exception (or maybe even include the list of allowable values for privileges in the exception, etc..).

[elasticmachine]

Original comment by @tvernum:

This is still an issue in 6.2
This:

{
  "test" : {
    "cluster" : [
      "manage_index_templates",
      "no-such-priv" <- This
    ],
    "indices" : [
      {
        "names" : [
          "test*"
        ],
        "privileges" : [
          "does-not-exist" <- This
        ]
      }
    ],
    "run_as" : [ ],
    "metadata" : { },
    "transient_metadata" : {
      "enabled" : true
    }
  }
}

Is considered to be a valid role.

[bizybot]

Also, we need to add debug log here and similar other places where required.