Added sonar.containerArgs argument so extra VM arguments could be passed to the internally started container

The argument is useful if someone wants to pass extra arguments to the container that is started internally. For example passing memory settings, javaagents (JRebel), etc.

Full disclosure: I work at ZeroTurnaround (www.zeroturnaround.com), the developers of JRebel and this development came from a customer request where someone wanted to use JRebel (specified as a javaagent to the VM) in his sonar development process using the sonar-dev-maven-plugin. However, I feel like this argument can be useful also in various other scenarios.