Allow 'splunk-url' param to be a list

[luckyj5]

Current behavior of docker-logging-plugin only accepts one value for splunk-url field. We should update the plugin to accept list for splunk-url.

[abhigarg28]

This is more of trying to understand the functionality here, does this plugin supports multiple HEC endpoints? If yes, could you please provide the link to the documentation on how to. If no, that means different logs will have to be send to the same sourcetype, which takes away the searching flexibility.