slapos-master: Switch to ZODB4-wc2

SlapOS Master is going to use Wendelin.core 2.
Wendelin.core 2 works with either ZODB4-wc2, or ZODB5, but not with ZODB4(*).
ERP5 does not currently work with ZODB5.
-> this leaves ZODB4-wc2 as the only choice.

(*) see ZODB@8e7eab33
for details.

/reviewed-by @tomo, @rafael
/reviewed-on !944
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:26:01

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:17:32

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

15:43:12

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

05:04:24

passed SlapOS.SlapRunner.ResilienceTest-Master

00:26:28

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:24:50