Several users have specified a desire for more control over data placement, e.g. to limit files within a directory to servers in a particular rack or to deal with heterogeneous brick sizes.  Almost all of this can be done from a script, with the caveat that the user's carefully calculated layouts will be stomped the next time rebalance is run.  Therefore, the minimal change we can make to support these needs is to define a layout type that behaves exactly like the one we currently have except that fix-layout (and thus migrate-data) will leave it alone.