[stable/redis-ha] set default haproxy image to alpine build, fix issu…

[martijnvdp]

…e #248.

What this PR does / why we need it:

change default haproxy image tag to alpine build as that build has less security issues

Which issue this PR fixes

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

• fixes #248

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])