Take Hiredis into use in Redis backend

Add dependency to Hiredis package to take it into use in ricsdl
Redis backend, because Hiredis provides better performance for
Redis server response handling than pure build-in implementation
of Redis client in python.

Add new unit tests for configuration reading from environment
variables.

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I43227df9df4f3a84a2646d0aaa71552d0105c9d5