Support authentication and SSL for redis sentinel (backport #1376)

[mergify]

This change allows enabling SSL and/or authentication for redis sentinel. Previously both of these were possible for only redis, and connections to redis sentinel servers were not protected.

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This is an automatic backport of pull request #1376 done by Mergify.

[mergify]

Cherry-pick of 20e5964 has failed:

On branch mergify/bp/stable/4.6/pr-1376
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	both modified:   gnocchi/common/redis.py

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