[bitnami/etcd] Non-manual chart upgrades

[fmulero]

Description of the change

Use common.secrets.passwords.manage helper to to lookup for existing passwords.

Benefits

Avoid manual intervention during upgrades.

Possible drawbacks

None

Additional information

Similar solution was applied to the rest of the charts.

Checklist

• [X] Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• [ ] Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• [X] Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• [X] All commits signed off and in agreement of Developer Certificate of Origin (DCO)