Improve environment setting

Some environment variable were set in the configuration files.
This wasn't a valuable solution. Allowing now system tning to
put environment variables in files of /etc/afm/unit.env.d
is much more flexible.

Change-Id: I1796061c9055a2860f0bbab340aa3a6e7b85dd3d
Signed-off-by: José Bollo <jose.bollo@iot.bzh>