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