Set MAC address via config file.

[rsteckler]

I'm running the container in Kubernetes. Every time the container is stopped and started, it gets a new MAC address, which breaks licensing. Kubernetes also doesn't allow one to set the mac address because the networking layer implementations don't support it.
This PR allows one to set the MAC_ADDRESS variable in the xeoma.conf. It's then set in the container before the xeoma server starts.

[skylord123]

I'm leaning towards getting rid of the config file for env variables. This would make things like upgrading easier (just change env variable instead of changing config file).

[rsteckler]

Cool. I would still suggest using the method here to set the mac address supplied by the --mac-address environment variable. It doesn't require bridge networking and it's all handled within the container.