This change allows uses to enable the healthcheck middleware from
oslo.middleware in API pipeline, by setting the [healthcheck]/enabled
option. This middleware provides an API endpoint at /healthcheck path
which allows load balancers or monitoring applications to determine
a service is up using HTTP requests.
This change basically follows the same change merged in ironic[1] repo.
[1] 6f439414bdcef9fc02f844f475ec798d48d42558
Change-Id: Ic2ee2bca74ea2a5a0723ef54b10c531f77ea7b8d