This patch fixes two bugs in Zookeeper backends:
* backend was determining if the node is a leaf or not basing on the number
of the childer given node has. This is incorrect if you consider the fact
that deleteing nested node can leave empty prefixes/dirs behind which have
neither children nor data inside. The fix changes this situation by testing
if the node has any data set - if not then it is not a leaf.
* zookeeper does not delete nodes that do not have childern just like consul
does and this leads to leaving empty nodes behind. In order to fix it, we
scan the logical path of a secret being deleted for empty dirs/prefixes and
remove them up until first non-empty one.
