Don't try to overwrite existing tagged artifacts

We currently will try to overwrite existing artifacts with rsync if they're
changed when we ship. However, we don't want to do this with tagged
artifacts, because they're immutable.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #192 from MosesMendoza/maint/master/dont_try_to_overwrite_tagged_artifacts

Don't try to overwrite existing tagged artifacts