curtin.util: re-add support for RunInChroot
In merge committed to trunk in revno 404, we dropped util.RunInChroot.
That method was being used from curthooks in lp:maas-images project to
run commands in target via util.ChrootableTarget. In order to support
installation of old images that depend on RunInChroot we need to add
the method back.
This merge does that by wrapping ChrootableTarget class. It also adds
a couple unit tests to ensure that this keeps working.