Debian package filenames don't follow the convention

As of Logstash 2.4, the filenames of published Debian packages isn't following the Debian convention of an underscore between the package name and the package version. Additionally, the 5.0.0-beta1 filename doesn't include the architecture name. Example URLs currently linked from https://www.elastic.co/downloads/logstash:
- https://download.elastic.co/logstash/logstash/packages/debian/logstash-2.4.0_all.deb
- https://artifacts.elastic.co/downloads/logstash/logstash-5.0.0-beta1.deb

These should've been logstash_2.4.0_all.deb and logstash_5.0.0-beta1_all.deb. Quoting [§7.3 of the Debian FAQ](https://www.debian.org/doc/manuals/debian-faq/ch-pkg_basics.en.html#s-pkgname):

> The Debian binary package file names conform to the following convention: _-_.deb

Start by tracing the package publishing configuration behind the two example download URLs and compare the generated names with Debian's filename convention in section 7.3 of the Debian FAQ. Done means the 2.4.0 and 5.0.0-beta1 packages use an underscore before the version and include the architecture name.

Written by the indexing model from the issue text.