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.

Related: rhbz#1036515

Change-Id: I837ca8fed04780224d4cadcb535728b1b1d2adc5