Output an error on unknown validation symbol

The dns filter contained the following `config :timeout, :validate => :int, :default => 2`
However as the `:int` validation does not exist, it did not convert string config value to int as encountered
in https://logstash.jira.com/browse/LOGSTASH-1650

This PR add a default branch to the case statement to report unknown validator symbol 
+ fix the dns using the correct `:validate => :number`