fix up cache problems

The first patch just starts caching the servers each time we do a show_instance on them. We do this several times in the build process anyway, and the extra step to just dump the contents to the file shouldn't be taxing.

The second one is to fix up the usage of delete_minion_cachedir, so that it actually goes all the way to where the file will be to clean it up.

Before it was just hitting /var/cache/salt/cloud/{active,requested}/<name>.json, now it adds the driver/provider top the directory tree as well.

Thanks,
Daniel