Attributes should also be set via environment variables

Currently, an attribute such as mcast_port=${my.mcast_port:7500} is set to 7500 by default. If there is a system property my.mcast_port, it will override the port.

We should also look for environment values; the precedence would be

• If there is a system property, use it
• If not, if there is an environment variable, use it // this is new
• Else use the configured value (e.g. 7500)