revert allow_mult default only for untyped (default type) buckets

as discussed.

confbal -- I attempted to update the description for `allow_mult` but am realizing now that this is probably insufficient given all default bucket properties in `riak.conf` only affect untyped buckets and we have yet to decide if "untyped buckets" is in fact what we want to call them.

Before Merge:

* [ ] verify test changes are correct and pass
* [ ] CSE +1
* [ ] PM +1
* [ ] Have a plan re: testing (specifically `riak_test`) as discussed -- ideally testing both setting values, or at a minimum preserve `allow_mult=true` in tests that have been updated to account for it