Added support for get_listable_tags() that optionally takes a metadata_key as parameter.  Returns a list of top level keys if no key is specified.  Added code to handle conditions in get_user_metadata() where  listable or non-listable metadata may or may not exist.
Added support for get_listable_tags() that optionally takes a metadata_key as parameter.  Returns a list of top level keys if no key is specified.  Added code to handle conditions in get_user_metadata() where  listable or non-listable metadata may or may not exist.
ab24v07/atmos-python,chadlung/atmos-python,MrBasset/atmos-python,lweeks/atmos-python