debian_buid.py: reset new_tag when build with autoincrement

debian_buid.py: reset new_tag when build with autoincrement

let's assume this:
- user changes files but not version in changelog and commits
- call_jenkins accepts and triggers build
- debian_build.py figures out version / tag to create from changelog
- debian_build.py will incrase version number and pushes new HEAD back to origin
- call_jenkins will not push trigger another jenkins call
- debian_build.py will build with the new version
- tagging must happen when build was successful BUT with the new version number
  instead of the one gathered in the very first beginning of the script