Add NaN handling config options
input/collectd: Implement handling of NaN values
Add tests for the collectd NaN handeling
input/collectd: Better tagging on NaN

  * Don't overwrite existing tags
  * Allow the user to config the tag to add when a NaN is found
Merge pull request #1015 from pieterlexis/collectd_NaN

NaN value handling in the CollectD input (LOGSTASH-1763)