Previous emptying of whitelist only affected callbacks that were
constructed for need whitelist. This now works for all callbacks.

This is simpler way to prevent persistent containers across instances
of classes that use field attributes

also moved the config param to a 'deprecated' list in constants.py
added TODO for producing a deprecation warning for such vars

prevents writing a play w/o a hosts entry which would default to
all/empty

This reverts commit 87969868d42cd8aba1c65c8207d059d73407373b.
found better way to do it

Galaxy 2.0

avoid persistent containers in attribute defaults