release_tarball: select the "v" tags for cephver

By default, Jenkins' Git plugin adds its own tag to the repository. This interferes with the versioning in `release_tarball.sh`

One solution would be to disable the tag in the Jenkins job. Since this is not the default, that tag creation option would need to be manually disabled for every job in Jenkins that runs this script. The other option is to make the script more selective about what it considers to be `cephver`. This pull request implements this option.

Use a --match selector to "`git describe`" so that we're sure we're using the standard "vX.X" tags.

Pipe "`git describe`" through "`sed`" instead of "`cut`", so that it's easier to debug what tag was selected.