[charts/redis-ha] fix haproxy auth config

[gi0baro]

What this PR does / why we need it

This PR fixes missing ENV var replacement in HAProxy config for slave backend in redis-ha chart.

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])

[edubacco]

I have the problem related to this PR. Any chance to get it merged? @DandyDeveloper Thanks