Move notification point to a better place

Follow-up for I80f325c9be9c171c2dc8d5526570bf64f0f87c78
"notify calling process we are ready to serve"

Xavier Queralt found issue with sending readiness notification from inside
run_service: it returns premature OK in case of multiple service endpoints
in one systemd service like nova-api which by default provides osapi, ec2 and
metadata services.

Instead, send readiness notification before entering wait loop for each
Launcher type.

Change-Id: I69bf10f353f5fecaaf89e537c2d3c46b73e39d36
Implements: blueprint service-readiness
(cherry picked from commit 356ecd4d9ab456f49c40cd6fb4e235fd18de0e19)