Flocker config resource limits floc 963

Fixes FLOC-963
Hmm, interesting comment. I agree that bytes is too granular to be a useful unit in the sense that people don't ever want to limit to a specific number of bytes rather than KB, MB or GB, but on the other hand, it's not difficult to convert (for example) 100MB to 100000000, probably a more natural conversion than working out 100MB in KB. The native Docker way is to assume bytes if no unit is specified, but also allow people to write 100M or 2G etc. If we're going to support only one unit for memory limits and therefore expect the value to be an integer, to me it makes more sense for it to be the base unit of bytes. But I think there's also a good case that we should accept string input in the 100M form, which should also be supported by docker-py (or we could internally always convert the supplied value to bytes, which is consistent with the format returned by the Docker API for an instantiated container).