Give an option to disable automatic tagging

Currently the Git plugin always creates an annotated tag at the start of the build. As a side effect it is not possible to use the "git describe --dirty" command to identify the Git version going in the build if we don't push back the tags at the end of the build.

We purposely don't want to push the tags created by Hudson, as there will be simply too many of them and adding no value. There might be cases where we'd want to do this but on rare occasions.

It would be very useful to add an option to disable the automatic creation of a Git tag at the start of the build. Right now this is not possible and handicaps or complicates our ability to easily and unambiguously identify our builds.