sbuild task: add pre-upload consistency checks on sbuild results (#94)

Consistency of files generated by sbuild with the files mentioned in .changes

• In the unit tests: replace write_changes_file(changes_file) by write_changes_file(changes_file, [file1, file2, fileN...]). The written changes file has the correct file names, sizes and hashes for the given files (needed so existing tests keep working) after adding validation checks
• Add ValidationFailedLog local artifact to send the log
• Sbuild class: new method _check_build_consistency that return a list with the errors (or empty list if no errors founds). Currently it checks that the files in the .changes exist and have the correct hash; and also checks that no extra-files were generated that are not part of the .changes