Add JSON params for caching arguments

* New option [cache] backend_arguments - is a JSON string,
allows to configure additional hashed params, e.g. behaviours
for dogpile.cache.pylibmc arguments.
* New option [kvs] arguments - ditto
  TODO address kvs part in tests

Co-Authors: Alexei Kornienko
Closes-bug: #1362173

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>