This change add several configuration options to enable HTTP checks
to the HAProxy configuration, instead of the default TCP connection
checks (which continue to be the default). It also enables /healthcheck
endpoint for heat-api and heat-cfn-api on openstack releases >= queens.
Closes-Bug: #1880610
Change-Id: I94c9418c82cdddd5a5d9ed400ab47889bfb225b1