Code review comment for lp://qastaging/~stub/charm-helpers/bug-1195649-fix-write-file

I don't see a lot of value in having write_file do anything magic so long as there's an easy way to do it elsewhere. host.py, as I see it, should simply make it easy to interact with the host and shouldn't care much about the fact that it's running in a juju environment.

That means we should probably clean up rsync(), symlink(), and mkdir() as well. I'll file bugs for those.

This looks good!