configuration: support default values with ${ENV_VAR:default_value} syntax

[muxator]

#3543 introduced suport for configuring via environment variables in settings.json.
The syntax was inspired to Elastic Serach's one.

It may be useful to extend this syntax, also supporting a default value in the configuration, so that if no environment variable is given, the default is used. For this, the model can be the following syntax:

To specify a default value, use:

${VAR:default_value}

Where default_value is the value to use if the environment variable is undefined.