Refactor service readiness notification

import and apply Oslo systemd module
drop old keystone.common.systemd
drop onready configuration parameter
 - systemd notification is no-op when not running inside systemd

Oslo commit 53e1214c092f09e3851b1a1b55289a93a72b09ec

Change-Id: I80f325c9be9c171c2dc8d5526570bf64f0f87c78
Merge "Refactor service readiness notification"