general: move get_backend to RateLimiter

* get_backend function is now part of the RateLimiter
  class.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
tests: removes flask-cache from dependencies

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>