Feat/Add redis sentinel dashboard

[slopezz]

Related to #196

• Adds a whole new redis sentinel grafana dashboard
• Adds sentinel controller suite testing (was one of the few controllers without testing)
• Updates a sentinel sdwon label (from server to redis_server), to be consistent with the rest of sentinel metrics
• Redis sentinel ashboard can be checked at staging

There are a couple of issues/bugs that will be opened soon with details:

• One related to most important counters whose default value is null, making the usage of needed rate to detect changes on a counter not usable for the first event since the operator runs (initial reported value should be always 0)
• One related to the role label, for every change on a role for a redis_server a new timeserver is created (OK), but the old timeseriesDB keeps being reported ad infinitum with latest retrieved value, instead of stop reporting the not existing metric, making the coexisting of lots of metrics which dont exists anymore

/kind feature
/priority important-soon
/assign

[raelga]

/lgtm

[3scale-robot]

LGTM label has been added.

Git tree hash: 4e0890d7b1ff3338aaedd2a4483ee1e785d8f42c

[slopezz]

/approve

[3scale-robot]

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