Implement Chocolatey build and refactor pipeline script

[bernd]

The build now creates a Chocolatey .nupkg file on every build and publishes the file when building a release.

Refactor the pipeline script to run all stages on the same node. This ensures that all stages have access to the build artifacts and we don't have to stash/unstash the files for each stage.

Additional changes:

• Update "graylog-sidecar.nuspec" file with latest URLs and author information
• Generate the "chocolateyinstall.ps1" file from a template for every build instead of overwriting the file for each release.
• Apply consistent formatting for the pipeline script

[bernd]

Review note: enable "Hide whitespace" on the diff to avoid the noise from the jenkins.groovy whitespace cleanup