xpack.[monitoring|management].elasticsearch.ssl.certificate_authority does not accept an array

*Original comment by @jpcarey:*

The `xpack.monitoring.elasticsearch.ssl.ca` is expecting an string. It should accept an array similar to the rest of our stack. `xpack.monitoring.elasticsearch.ssl.ca: []`

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[2017-03-06T12:13:24,786][FATAL][logstash.runner ] An unexpected error occurred! {:error=>#, :backtrace=>["/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:208:in `validate'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:384:in `validate'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:394:in `validate'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:171:in `set'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:61:in `set_value'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:80:in `merge'", "org/jruby/RubyHash.java:1342:in `each'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:80:in `merge'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/settings.rb:115:in `validate_all'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/runner.rb:210:in `execute'", "/Users/jared/builds/logstash/logstash-5.2.2/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'", "/Users/jared/builds/logstash/logstash-5.2.2/logstash-core/lib/logstash/runner.rb:183:in `run'", "/Users/jared/builds/logstash/logstash-5.2.2/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'", "/Users/jared/builds/logstash/logstash-5.2.2/lib/bootstrap/environment.rb:71:in `(root)'"]}
`