deb: add systemd service config for upcoming Jessie

This change adds a systemd service configuration file, and adds systemd logic to installation and de-installation scripts. The upcoming Debian 8 "Jessie" release will use systemd.

This means the elasticsearch .deb package will now ship both SystemV init and systemd configuration, and use whatever is appropriate on the target system. Tollef Fog Heen recommended this procedure on the  pkg-systemd-maintainers Debian mailing list (http://lists.alioth.debian.org/pipermail/pkg-systemd-maintainers/2014-December/005077.html).

fixes #8493