Code review comment for lp://staging/~wallyworld/juju-core/image-metadata-defaults

code wise, this looks mostly mechanical changes due to a new config
item. And then moving the selection code into getInstanceType rather
than FindInstanceSpec, which seems fine to me.

So the code changes are good, I'd just like to discuss our specific
naming to make sure we aren't adding complexity that won't actually be
used.

https://codereview.appspot.com/9303051/