Dimension keyword name

@drsnyder 
don't let colons get into the filename in CEPH. Some clients (Boto in Python) have trouble reading these files.
Should we do this with everything that we translate to a string in the filename? E.g. [here](https://github.com/Wikia/vignette/blob/dimension-keyword-name/src/vignette/media_types.clj#L104)?