Xero for WooCommerce

When working in multiple environments, such as dev/test/production, each environment may have its own path to store the pub/private keys.

Having the fields in the Xero settings requesting an absolute path does not work for container-based services such as Pantheon.

We would like to have a way to reference environment variables for paths, or a way to override the path settings in the Xero config. This could be acceptable by either by a filter or a constant.