Add (experimental) support for systemd as an `init_style`

First stab at getting `systemd` to work. Still not sure about a few
things such as using an EnvironmentFile and the code for setting up
everything related to init styles is even messier, so it could probably
use some cleanup and rethinking.

But hey, it works.
Fixup of env. file for systemd

Add explicit filename for the log4j and server properties, cleanup
unused ERB tags and fix JXM_PORT tag.
Add `systemd?` helper for integration tests

The version of serverspec that is used is unable to check whether a
service is enabled for distros that use systemd, so until there’s a
better fix for it this’ll have to do.