This change keeps the previous behavior but implements it using multiple
pipelines (pipelines.yml) instead of with `path.config`.
* The default pipeline name is still "main"
* Default config path is still /etc/logstash/conf.d/*.conf
Other small changes included:
* Specify fpm workdir. Otherwise rpmbuild is given an empty value for
its temporary directory. It's unclear if this is a bug in fpm or in my
environment, but I like the change anyway to put fpm's build activity
in the build directory.
* DRY's up the common files going in /etc/logstash for both rpm and deb
packages.
For #8893Fixes#8894
