Implement ROSETTA_CACHE_NAME, which defaults to 'rosetta' with …

...a fallback to 'default' if unset.

Hi,

In my environments I need the dummycache on the development/test side to avoid cache display issues, and I don't intend at all to provide rosetta on production environments on which I don't want to mess with git merges, etc.

I thus hacked rosetta to be able to use a dedicated cache, if present in the django settings. It will automatically use the `rosetta` cache if defined. But for more complex setups the app will lookup a new setting which defines the cache name.

On default and simple installs, it can simply stay unset and the app will happily use the `default` cache. 

Your current tests against dummycache remain completely valid and will test the final destination to avoid typos / other human config errors.

In the patch there are also small flake8 happyness modification, like unused imports removed or "line too long" wrapped. Nevertheless, I didn't alter the whole project to fix flake8 suggestions everywhere. Just drop me a note I you would like me to.

Hope this patch will make it in rosetta mainline. Best regards,

--
Olivier