Refactor service readiness notification

Build upon cceda95a35f18f5f7b52daaf0662a4cd3768b3ac apply Oslo systemd
module. It was imported in aef33d2d716c064f1f571c334fd36b1ea5928d5c. It
also drops heat.common.systemd and deprecates onready configuration
parameter.

Oslo commit 53e1214c092f09e3851b1a1b55289a93a72b09ec

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