[charts/redis-ha] Add a preStop script to trigger a failover before a redis master is terminated

[adyxax]

What this PR does / why we need it:

I noticed that when a redis master gets terminated there is a downtime before a new one gets elected. This behavior can be fixed with a preStop script that checks if the terminating pod is a redis master and triggers a failover before shutting down if that is the case.

Special notes for your reviewer:

Do not hesitate to ask for clarifications or changes. I tried to respect the style of the surrounding code.

Checklist

• [X] DCO signed
• [X] Chart Version bumped
• [X] Variables are documented in the README.md
• [X] Title of the PR starts with chart name (e.g. [stable/mychartname])

[adyxax]

@DandyDeveloper : Please review this PR when you have time, thank you!

[DandyDeveloper]

@adyxax Great idea. I like this.

I'll jump on the review later today. One immediate thing. This should be a minor increase. Not a patch increase.

[adyxax]

Thank you, I updated the version to a minor increase.

[adyxax]

@DandyDeveloper : if I can do anything to help with your review do not hesitate to ask.