Configuration using environment variables

[JonathanKnam]

Hi,
you might want to fix the documentation for the usage of environment variables in the wiki: Environment Variables.

The configuration should be: ${ENV:YOUR_ENVVAR_NAME} instead of ${YOUR_ENVVAR_NAME}
Same in the example xml: ${ENV:HIVEMQ_PORT} instead of ${HIVEMQ_PORT}

I've also noticed, that environment variables will be written as replaced variables to the config, if wiki: Mutable Configuration is enabled.
I'm not sure if this is a desired behaviour.