Feature: Configuration Option for Dollar-Topic Publishing

[Z0rdak]

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Motivation

I've described the motivation in my previous created issue, but never got an response.

Please let me know if any further changes need to be done.

Resolves #523

Changes

In short: Removed old internal constant to allow clients to publish to dollar topics and replaced it with a XML configuration entry.

Listed changes mostly reflect commits:

• add new boolean configuration for allow-dollar-topics
• add constant for allow-dollar-topic default value
• extend MqttConfigurationService interface with getter and setter for new config option
• add implementation for new interface methods
• add config entity entry to MqttConfigurator
• adapt tests to include new config entity
• remove old internal configuration option constant
• add MqttConfigurationService as argument to PluginAuthorizerService
• adapt PluginAuthorizerService test