easier plugin testing without outputs

[rubycut]

I should be able to test plugins without without any outputs, that is the whole point of testing, currently it is not possible.

telegraf -test -config-directory . -input-filter http_response
2016/09/28 10:14:42 Using config file: /etc/telegraf/telegraf.conf
2016/09/28 10:14:42 Error: no outputs found, did you provide a valid config file?

[danielnelson]

Implemented in #2446