[Review Queue] storage and zabbix-agent (personal namespace) followup

[Cory Johns]

https://code.launchpad.net/~tribaal/charms/precise/storage/refactor-mount-volume/+merge/232236

I had some back-and-forth discussion with Chris and David regarding
this refactor, which looks like a great change to me.  I have
personally reserved my +1 due to some tests that were necessarily
removed from the `make test` target due to this change, but Chris
makes a good point that reintegrating them will further complicate an
already complex review, and it appears that he and David have
discussed it at length.  So my review was more of a +0 than a -1
(pending an issue with testing this with the postgres charm that David
uncovered).

https://bugs.launchpad.net/charms/+bug/1369892

I did a follow-up on my previous review for Samuel.  I felt like there
was still some lack of clarity regarding how personal namespaces work,
so I tried to explain it further, with links, as well as responding to
Samuel's direct points.  This highlights the need to better describe
in the documentation the distinction between Recommended charms and
personal namespace charms, as well as precisely when a review is
required vs when one can be optionally requested simply for feedback
or advice.