compositor: Avoid loading systemd-notify plug-in

compositor: Avoid loading systemd-notify plug-in

This helps being compliant with the weston ini configuration file, and
in the same time being able to load other necessary modules. Instead of
creating a separate module for loading the systemd-notify do the same
thing as xwayland and avoid any attempt of loading it if passed over the
command line, or specified in the ini file.

Bug-AGL: SPEC-4001

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I46018f2dff5599bb3acc7c7334e77437995d0588