Pre-flight check and yaml_diff tools

The pre-flight check tool detecta and fix configuration problems. The yaml_diff tool checks for differences between two cassandra.yaml files.

The pre-flight check tool, located in /usr/share/dse/tools of packaged installations, is a collection of tests that can be run on a node to detect and fix a configuration. The tool can detect and fix many invalid or suboptimal configuration settings. The pre-flight check tool is not available in tarball installations. 

The yaml_diff tool that filters differences between two cassandra.yaml files, which is useful during upgrades. The new tool is located in the tools directory of tarball and packaged installations.