Alerta does not pick up on non-default Postgres credentials

[davidusken]

Issue Summary
When using credentials other then the default ones for Postgres, Alerta does not use them.
This results in having the password in cleartext in Git when wanting to use non-default credentials.

Environment

• OS: Linux

• API version: 9.0.1

• Deployment: Docker

• Database: [eg. MongoDB, Postgres]

• Server config:
  Auth enabled? Yes
  Auth provider? OpenID
  Customer views? No
  (or provide the /config endpoint output)

• web UI version: 9.0.1

• CLI version: 9.0.1

To Reproduce
Steps to reproduce the behavior:

1. Update alerta-postgresql with different credentials
2. Re-deploy app
3. Verify auth fails for alerta user as credentials alerta:PG_ALERTA_PASSWORD no longer works

For web app issues, include any web browser JavaScript console errors.

Expected behavior
Best case scenario the database URL should read from the secret.
Alternatively it should work to supply the new password as an env variable and use this syntax:
DATABASE_URL = "postgresql://alerta:${PG_ALERTA_PASSWORD}@fmonitoring-alerta-postgresql:5432/alerta"
(Tested this and it does not use the env variable to authenticate)

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Additional context
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