[packaging] flag files in /etc as configfiles

This prevents them from getting overwritten when the logstash deb/rpm is
upgraded.
[packaging] minor RPM improvements

 - add time and commit id to package revision number
 - include url and description
[packaging] install only one copy of logrotate.conf
[packaging] depend on default-jre-headless

On debian/ubuntu, "default-jre" will pull several audio and GUI libs by
dependency, which aren't required to run logstash.
[packaging] adapt previous patchset to new .VERSION.mk

To recap, the goal is to keep the commit sha1 in the package release
number after the always-incrementing timestamp, and not in logstash's
version number.

Otherwise, we risk having dpkg/rpm believe the package will be
downgraded in the case let's say you have an 1.2.3.dev-abc123 version
which arrives after 1.2.3.dev-def456.
Merge branch 'packaging-conffiles-and-rpm-improvements' of https://github.com/mfournier/logstash into mfournier-packaging-conffiles-and-rpm-improvements

Conflicts:
	Makefile