REVIEW-2194: change the buildsetup integtests to check more the 'what' instead of the 'how'.
REVIEW-2194: rework and simplify BuildSetupPlugin. Moved the logic to choose and apply correct project descriptor in SetupBuild task. Besides this task only wrapper task is applied by BuildSetupPlugin

- deleted GenerateSettingsFile
- deleted GenerateBuildFile
- deleted ProjectLayoutSetup
- kept ConvertMaven2Gradle for now (maybe there are usecases to keep that logic in a seperate task)
Update userguide chapter of buildsetup plugin after changes related to REVIEW-2194
REVIEW-2194: Add hint to supported build setup types in error message.