Add systemd notify support

Add systemd notify support

The drm-lease-manager create unix domain socket file to provide lease to
lease client.  In typical case, this file must be create before client
boot up.  But existing drm-lease-manager couldn't  keep that sequence
because that support startup ordering only using systemd service dependency.

This patch add systemd notify support to realize more strictly ordaining.

Bug-AGL: SPEC-4427

Signed-off-by: Naoto Yamaguchi <naoto.yamaguchi@aisin.co.jp>
Change-Id: Icaad6c9df0696a48b9534c7586be7949a874f9a5