Fixes Bug 999060 - mixins inadvertently propagate changes to other classes

I've discovered that the mixins are overworking.  If we have jobs A, B  and C, at they, repetitively require resources X, Y and Z, the mixins will set it up like this:

A -> X
B -> X, Y
C -> X, Y, Z

when it should be:

A -> X
B -> Y
C -> Z

this happens because the mixin decorators are using the 'required_config' directly rather than calling the 'get_required_config' method.  This is a relatively benign problem as jobs with extra resources just ignore them.  It makes for messy ini files, though.