Doc: Set version attribute in plugin source file

[karenzone]

Related to plugin versioning work in https://github.com/elastic/docs-tools/pull/70 and https://github.com/elastic/docs-tools/pull/73. The stack version attributes must be (calculated and) set on a per file basis, as the stack version will increment over time. I believe that this approach represents the right place to set those attributes.

Notes

• I've used conditionals to ensure that the version attributes are set for the Versioned Plugin Reference only.
  • Why? The Logstash Reference is stack versioned, and we don't want the VPR approximations to interfere by causing unintended consequences.
• When tested and approved, these attribute settings need to be added for each plugin affected by versioning problems (output-elasticsearch, input-beats, etc.) Don't worry... I know the list. 😄
• When the versioning work has been implemented and fully functional, the VPR will need some cleanup to remove past "band-aids" to versioning.
  • When the cleanup work is complete, I'll remove the attribute resets at the end of each source file.

[karenzone]

cc:/ @mashhurs

[karenzone]

PUBLISHED: https://rubygems.org/gems/logstash-input-file/versions/4.4.1