stack/erp5: make haproxy reload script wait for old worker processes

This is mostly to fix some flaky tests, because with the previous
approach of just sending a signal for haproxy to reload the config
asynchronously we don't know when haproxy has reloaded.
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:25:33

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:23:02

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

10:40:04

passed SlapOS.Theia.ResilienceTest-ForTestRunner1-ERP5

07:49:53

passed SlapOS.Theia.ResilienceTest-ForTestRunner1-Gitlab

12:41:40

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:23:03

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:22:56