[bitnami/jenkins] Add Jenkins Password Secret and Secret Key Values

[speedythesnail]

Description of the change

This change enables Jenkins to use an existing password secret, rather than having to provide a password in the chart.

Benefits

This change enables better GitOps by allowing people to provide an existing secret name, rather than having to put a password in their custom values. The values file would then be able to be safely stored in a git repository, without the risk of secrets being exposed.

Possible drawbacks

None known, this change should be backwards compatible with the existing chart as it changes no defaults.

Checklist

• [X] Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• [X] 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)