Add Redis Sentinel config as environment vars

[NeodymiumFerBore]

Related Issue: None

New Behavior

Allow to configure Redis Sentinel via environment variables. This change does not break existing configurations.

Contrast to Current Behavior

Cannot configure Redis Sentinels without using extra config mechanism.

Discussion: Benefits and Drawbacks

If environment variable REDIS_SENTINELS or REDIS_CACHE_SENTINELS are not defined, the resulting configuration will be an empty list []. Upstream netbox also defaults it to [] (redis tasks, redis cache).

I chose to not default REDIS["cache"]["SENTINELS"] to redis tasks Sentinels (as it is done for HOST and PORT for example), because SENTINELS always takes precedence over direct connection. And one may want to use Sentinels for a redis instance but not the other one.

Benefits:

• More configuration via environment variables, without extra configuration
• Does not break already existing configurations

Changes to the Wiki

I don't think any

Proposed Release Note Entry

Allow to configure Redis Sentinels via environment variables

Double Check

• I have read the comments and followed the PR template.
• I have explained my PR according to the information in the comments.
• My PR targets the develop branch.

[NeodymiumFerBore]

Just realized PR #1130 was already opened for this topic, but as draft. Since this one has less code duplication, I keep it open.