Use environment variable for URLs

[Sven Lieber]

In case someone does not want to put the publicly available URL of the SRU endpoint into the config, a workaround has to be used.
The current workaround is to make the connection type authenticated and use the following configuration:

"connection": {
    "type": "authenticated",
    "url": "$userVariable",
    "userVariable": "ENV_URL",
    "passwordVariable": ""
}

However, it would be more straightforward to make the usage of an environment variable for the URL possible too.
Authentication is a different concern.