Use checked commit when committing kazoo transactions

To avoid the case where the transaction appears to have
committed successfully (when it actually has not) create
a new checked_commit utility function and use it in the
zookeeper persistence backend to ensure that the transaction
has completed correctly (when it has not raise a new kazoo
exception that contains the failures to the caller, which can
then reraise that exception, examine the failures or do some
other action).

Change-Id: Ib169b36abb568a0e1516c4c786fccb30856661e1
Merge "Use checked commit when committing kazoo transactions"