overrideable property for bitops schemas

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

Start with scripts/helm/bitops.schema.yaml, then trace how bitops.config.yaml and the Docker environment are read. Verify the schema supports an overrideable or import_env property and that an existing NAMESPACE value takes precedence over the configured namespace when running the BitOps image.

Written by the indexing model from the issue text.