This introduces support for Redis Sentinel backend. Users can now
use Redis Sentinel backend instead of Redis backend by configurations
like the example below.
[cache]
enabled = True
backend = dogpile.cache.redis_sentinel
redis_password = <password>
redis_sentinels = 192.0.2.1:26379,192.0.2.2:26379,192.0.2.3:26379
If tls_enabled option is set to True then all the tls settings are
applied for connections to Redis as well as connections to Redis
Sentinel.
Change-Id: Ic3b84fe6810e08337a884c68625ccfed11665269
7bb43bbbd5