feat: added support for flag overrides using environment variables

[kschjeld]

Description

Added support for overriding Grafana image/tag and plugins image/tag through environment variables.

This is needed to allow configuration overrides when installing the operator in OpenShift 4.x using OLM, as the Subscription resource doesn't allow for adding command line arguments to the deployed operator.

Type of change

• [x ] New feature (non-breaking change which adds functionality)

Checklist

• [ x] This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

In OpenShift when deploying using Subscription, add a section as follows:

 config:
    env:
    - name: GRAFANA_IMAGE_URL
      value: someregistry.com/somegrafana

Verify that the Grafana deployments created uses the specified container image url.

[HVBE]

Awesome work! thanks for addressing this, and the neat documentation to go along with it :) @pb82 @NissesSenap thoughts?

[pb82]

thanks @kschjeld