stack/erp5: Add experimental per-computer hosts overrides.

Use-case: when an host enry is used to define the outgoing binding IP of
zopes in a partition, this IP is host-dependent.
9 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:14

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:17:55

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

08:38:42

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:47:58

passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:44

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:25:58

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:22:11

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

07:53:50

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:41:42