Add warning message to error log when starting Galera without RBR

[dtest]

Running Galera in anything other than RBR can cause some unintended consequences. There may be reasons to do so, but in general, best practice is to run in RBR.

This is a feature request to add a warning to the error log on startup if binlog_format != row. Administrators can chose to ignore it, but this helps add a reminder in case it was something they overlooked.

Discussion: https://groups.google.com/forum/#!topic/codership-team/XpldSuAms5o