Code review comment for lp:~rogpeppe/juju-core/502-upgrades-simplify

AFAICS, this just makes the code harder to change. Opinions may
reasonably differ re likelihood of changes that'd cause the interface
implementation to pay off, but IME the aggregated cost of designing for
change is trivial compared to the cost of failing to do so, and so a CL
that just removes flexibility does not lgtm.

https://codereview.appspot.com/63530048/