Remove display system specific environment

We need the prelaunch daemon to run in both X and wayland environments,
so directly exporting DISPLAY or ELM_ENGINE in the service file will
not work.  Instead the service file will use the EnvironmentFile mechanism
to load a snippet (if the file exists).

Each vertical can create a /etc/sysconfig/wrt file containing key/value
pairs that needed for the specific environment.  This could be done in the
post section of a ks file or any other mechanism.Z

Change-Id: I8e14233595c6e992b718ae9dfa14077a026c657c