Comment 6 for bug 1431194

As I mentioned in bug 1444841, two straight forward ways to solve this problem:

1. Keep instance.az, but make it obsolete. Set instance.az when booting, and always read host's az after that.

2. Make instance.az be consistent with host az. That means we always need to update instance.az in db, when host az changes.

The patch that I currently making is #1. Hope other good ideas.