Fix redis sentinel change myid every time when it restarted

If you use COPY_ALWAYS, kolla will overwrite /etc/redis/redis.conf.
But Redis Sentinel will store myid in this file. So when it restart,
Redis Sentinel will regenerate new myid every time. Redis Sentinel
cluster will see more and more sentinel, This patch use hash value
with hostname as sentinel myid.

Change-Id: I6d9403231e41baac177f29c58e67ba605795385d
Closes-bug: #1788179