[charts/portal]Changes to use existing DB secret that contains the DB password

[ksaladi]

Description of the change

Changes to use existing DB secret that contains the DB password.

Benefits

Enables customers not to expose the DB password from values override files.

Drawbacks

Applicable issues

• fixes #

Additional information

Checklist

• [x] Chart version bumped in Chart.yaml according to semver.
• [x] Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [charts/gateway])
• [x] If the chart contains a values-production.yaml apart from values.yaml, ensure that you implement the changes in both files