Simplify and optimize az server output extension.

Displaying two different az results to users is confusing, so just
return the az that the instance is actually in. Also, cache the
result from looking up the az by host for an hour to avoid making
lots of expensive db requests.

DocImpact

Change-Id: Ib39bf94c608874695aab00e61035e64f9594a985
Merge "Simplify and optimize az server output extension."