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 nexedi/ZODB@8e7eab33
for details.

/reviewed-by @tomo, @rafael
/reviewed-on nexedi/slapos!944

software/slapos-master/software.cfg

@@ -6,6 +6,15 @@ extends =
 parts +=
   vifib-fix-products-paths
 
+
+# SlapOS Master needs 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.
+[ZODB]
+major = 4-wc2
+
+
 [local-bt5-repository]
 # Same as bt5-repository, but only local repository.
 # Used to generate bt5lists.