improve systemd onready notification

* add unset_environment parameter
New parameter unset_environment was added to sd_notify
http://www.freedesktop.org/software/systemd/man/sd_notify.html
to ensure service readiness is sent only once.

* add onready() method to simulate systemd environment
For testing purposes and optional use with SysV initscripts.

Resolves: rhbz#1058291

Upstream-Oslo: https://review.openstack.org/72683
Change-Id: I80f325c9be9c171c2dc8d5526570bf64f0f87c78