[master+ZODB4-wc2] ZODB: Switch to using ZODB4-wc2 by default

It is handy to have such default in slapos@master+ZODB4-wc2 and/or
slapos@1.0+ZODB4-wc2 to setup ERP5, NEO, Wendelin, ... testsuites that
will test software wrt ZODB4-wc2 instead of ZODB4.

component/ZODB/buildout.cfg

@@ -15,10 +15,10 @@ parts = ZODB/scripts
 #   [ZODB]
 #   major = <ZODB-version-major>
 #
-# By default ZODB4 is used.
+# By default ZODB4-wc2 is used.
 [ZODB]
 recipe = slapos.recipe.build
-major  = 4
+major  = 4-wc2
 init =
   # link/depend ZODB -> ZODB<X>
   zodb_x = 'ZODB'+options['major']