Code review comment for lp://qastaging/~thedac/charms/precise/block-storage-broker/configure-repository

I've also got conditionals on the series, so the helper should one day end up in charm-helpers (core/host.py probably). I'm also doing something similar in the PG charm, requiring icehouse with precise, and have the same approach. I would like to know the difference between cloud:havana and cloud-archive:havana, and when one should be chosen over the other.

This branch is good to land. I'd like a comment on the 'null' replacement, as this is unobvious, but as the entire helper is uncommented it wouldn't help much.