Make cassandra-stress able to read all credentials from a file

If there is username/password required in order to connect via JMX, there is currently no way how to do this as NodeProbe constructor in cassandra-stress does not accept any credentials.

The way how we are reading the credentials should be refactored. Currently, if there are credentials on the command line, they will be visible in the logs. Making it visible on the command line for JMX credentials is not ideal either.

What I would like to see is to read all credentials which are necessary for cassandra-stress from ONE file. CQL and JMX combined.

Because there is already some logic in place and it would be cool to have this backward compatible, we may still support command line credentials for CQL but we would make it deprecated and we would remove it in 6.0 so cassandra-stress will be reading credentials from the file only.

I looked into the implementation and I have an idea how to "inject" credentials where necessary so they would be used even we do not use them on the command line but I have not coded up anything yet.