builder/amazon/chroot: alphebatize the configs

/cc @mwhooker
builder/amazon/chroot: CommandWrapper

/cc @mwhooker - I changed the interface up a bit to return an error,
since things should return errors in Go (the ui.Error bit was kind of
ghetto because it had no way to bubble that error up except through the
UI).

Using this, I made it so that the communicator uses both a
CommandWrapper and ShellCommand with chroot so that the chroot commannd
is also wrapped (it wasn't before).

I think the functionality of all this is the same but I'd love if you
could look it over and make sure.