Add node.config to debian

[jadeallenx]

Problem to solve: We need a mechanism to store/preserve specific configuration overrides for nodes. The configuration needs to be marked as such by the package manager so that the configuration remains durable across package installations.

Solution: This etc/node.config file enables durable, node-specific configuration. Because the file is considered/merged last, the terms in this file will override other settings that have been processed earlier during application start up. For more information see the Erlang sys.config documentation.

To enable this behavior in a Docker container, there is an option to use a bind mount using the command line -v option (example: -v /pathto/host-node.config:/var/helium/miner/etc/node.config)