fixing support for an external postgresql database with helm deployment

[shortcipher3]

Motivation and context

The helm chart documentation describes steps for using your own postgresql database as follows:

I want to use my own postgresql/redis with your chart.
Just set postgresql.enabled or redis.enabled to false, as described below. 

When redis.enabled is set to false the environment variables are changed in the cvat_backend/deployment.yaml, but when postgresql.enabled is set to false the postgres environment variables are not changed.

I made a small change that reads the settings from the yaml file for the external database.

How has this been tested?

Built with enabled set to false without the change and examined the environment variables of the cvat_backend service. Then applied my changes and rebuilt and re-examined, confirmed the variables were using my settings with my code changes, but did not use my external db settings without my code changes.

To check my environment variables I ran:

$ HELM_RELEASE_NAMESPACE="<insert>"
$ HELM_RELEASE_NAME="<insert>"
$ BACKEND_POD_NAME=$(kubectl get pod --namespace $HELM_RELEASE_NAMESPACE -l tier=backend,app.kubernetes.io/instance=$HELM_RELEASE_NAME -o jsonpath='{.items[0].metadata.name}')
$ kubectl exec -it --namespace $HELM_RELEASE_NAMESPACE $BACKEND_POD_NAME -c cvat-backend-app-container -- bash
$ env | grep -i postgres

Also without these changes I was unable to successfully run:

kubectl exec -it --namespace $HELM_RELEASE_NAMESPACE $BACKEND_POD_NAME -c cvat-backend-app-container -- python manage.py createsuperuser

I was getting a database error.

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[azhavoro]

@shortcipher3 Hello, thanks for the contribution! Could you please add a note to the changelog file about your change?

[shortcipher3]

@shortcipher3 Hello, thanks for the contribution! Could you please add a note to the changelog file about your change?

done!

[nmanovic]

@shortcipher3 , thanks again! Your contribution into the project is awesome. 👍