Have package installers install logstash.yml.dist

Ref #6073 from @dcode

When upgrading RPM/DEB packages, the installed file may not be overwritten. In such cases, we should change the pre-install script (or after-install script, wherever it's easier) as follows:
1. If there is no logstash.yml, install the package version and a copy as logstash.yml.dist
2. If there is an existing logstash.yml, and there is no logstash.yml.dist, do not overwrite logstash.yml, but install logstash.yml.dist
3. If both logstash.yml and logstash.yml.dist exist, overwrite logstash.yml.dist with the package version, and leave the existing logstash.yml alone.

I believe we can squeeze this in for 5.0 release since it is package-level and does not affect existing Logstash code.