overrideable property for bitops schemas

[mickmcgrath13]

originally mentioned here: https://github.com/bitovi/bitops/issues/136#issuecomment-1005850857

It would be nice to have an import_env or overriddeable property to the schema to essentially say if the export_env variable is already set, use it's value instead of what is defined in bitops.config.yaml https://github.com/bitovi/bitops/blob/master/scripts/helm/bitops.schema.yaml#L7-L11

This means we could do

docker run \
-e NAMESPACE="foo" \
... \
bitops:latest

and it would ignore what is in bitops.config.yaml's namespace: config