As we've enabled /healtcheck URIs for services [1], that we want to
verify, we need to apply haproxy backend changes only when all services
are re-configured, otherwise haproxy will fail to poll them and
mark all backends as down, which would mean continious downtime until
corresponsive roles will run to fix service api-paste configuration.
[1] https://review.opendev.org/q/topic:osa/healthcheck
Closes-Bug: #2002645
Change-Id: I004f1a86b4dba3c6e356ac14bb70a43abc17f538
befd8424e2