[stable/redis-ha] add maxUnavailable value for the ha-proxy rollingUpdate config

[martijnvdp]

What this PR does / why we need it:

This PR add a new variable .Values.haproxy.rollingUpdate.maxUnavailable with default value 1 this is needed to be able to restart the deployment on a cluster with 3 or less nodes

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #249

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

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