Gateway: GatewayService should register cluster state listener before checking for current state

At the moment we may miss a state change and fail to recover on time.

Closes #8789
Debian package: Prevent init script from returning when the service isn't actually started.

Close #6909
Docs: Added a command to start elasticsearch on bootup on Debian.

Close #8600
Docs: Add instructions to start elasticsearch on bootup on RHEL/Fedora.
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.

fixes #8943

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>