Add general environment variables option to support PGADMIN_CONFIG_ overrides as per the container docs

[patkearney]

What this PR does / why we need it:

Adds the ability to set arbitrary environment variables. Given the extent of options that could be overriden in config.py this seemed preferable to exposing each individually.

I was initially unable to deploy a default installation with a LoadBalancer against an AKS cluster. No user credentials worked when accessing via the LoadBalancer IP. There is a note in config.py as per disabling enhanced cookie protection within dynamic IP environments such as Kubernetes.

Which issue this PR fixes

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] DCO signed
• [x] Chart Version bumped
• [x] Variables are documented in the README.md

[alexnuttinck]

Hello @patkearney,

Thanks for this improvement! It will be helpful.