Support redis sentinel configuration

Based on environment variables set, sdl will select either to connect to
standalone redis or sentinel redis. The underlying redis-go
implementation takes care of how connection to master is handled and
what happens if there is a switcover in redis.

Change-Id: Ib413b61e6a7e26e8ad2bd7f83331554271a93c69
Signed-off-by: Marco Tallskog <marco.tallskog@nokia.com>