notify calling process we are ready to serve

Systemd notification should be sent in-process, otherwise systemd might
miss the subprocess sending notification.
See systemd bug https://bugzilla.redhat.com/show_bug.cgi?id=820448

Taken from keystone project commit
abc06716d027d68f0da3b0f559fa7c85a21804d5

Improvements from Keystone version:

    * 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.

    * unit test added

    * docstrings for notification methods

Patch includes deployment in openstack.common.service.
It does not have an effect when running the service outside
the systemd environment.

Implements: blueprint service-readiness
Change-Id: I80f325c9be9c171c2dc8d5526570bf64f0f87c78