stack/erp5/instance-erp5.cfg.in: Cleanup "directory" section.

Remove unused "plugin" entry.
Add locally-referenced-but-defined-by-another-file "bin".
4 jobs for 1.0 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:27:18

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:25:17

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3