Add toString method to TOSCA YAML model classes

[kleinech]

Add toString method to TOSCA YAML model classes

Change simplifies debuging output (e.g. why are yaml test working on travis but not on jenkins)

Signed-off-by: Christoph Kleine kleinech.github@gmail.com

• Ensure that you followed http://eclipse.github.io/winery/dev/ToolChain#github---prepare-pull-request. Especially, we require a single commit
• Ensure that the commit message is a good commit message
• Ensure to use auto format in all files
• Change in CHANGELOG.md described
• Tests created for changes
• Screenshots added (for UI changes)