Adopt `freeze-versions` from ///repronim/containers as `containers-freeze` (or `-copyconfig`?)

I do not think there should be anything specific to //repronim/containers in the 

https://github.com/ReproNim/containers/blob/master/scripts/freeze_versions

which at large just copies .datalad/config entry from subdataset into superdataset and adjusts paths.

There it is also for 'local freezing' so 'datalad update' would cause conflict if version gets changed. Again -- nothing specific to that.