Allow environment variables in node_service_prefix config.

[magnusfeuer]

The config file's node_service_prefix is used to specify the string to prefix all locally connected services with to make them globally unique.

Today this is a static string.

It should support integrated Linux environment variables, allowing the prefix to be dynamically set depending on what the environment variable is set to.

Now we have a statically configured value.
jlr.com/vin/saw813433993cv4/

Instead we should have:
jlr.com/vin/$env{VIN}/

Where $env{VIN} is replaced by the value of the VIN environment variable.

We should also support file content:
jlr.com/vin/$file{/etc/vin}
Where $file{/etc/vin} is replaced by the single line stored in /etc/vin