Add systemd notification support to heat

This mirrors work done in other OpenStack projects
to add systemd notification support to signal that
a given process is ready to accept requests.

The heat-api, heat-api-cfn, heat-engine, and
heat-api-cloudwatch processes are all covered by
a new, single, 'onready' configuration file option.

Additionally, the sample configuration file has been
updated to match.

Implements: blueprint systemd-integration

Change-Id: I90d2b915e2568a65ed9ed78923c9982e19f9c948
Signed-off-by: Lon Hohberger <lhh@redhat.com>
Merge "Add systemd notification support to heat"