MT#6751 use reports/tag.info file to check git diffs.

Workspace is volatile on slaves. We need to maintain a tag.info file with
the info of the building tags. This files needs to be copied from the master
node to the slave and copied back afterwards.
MT#6751 don't exec by default if no changes are found.

cppcheck_force environment variable with something != "no" will
force the check if no changes are found. This check is for ALL files.
MT#6751 split cppcheck job
MT#6751 use source environment variable
MT#6751 add source parameter to cppcheck* jobs