Add --no-push to catkin_prepare_release

The `catkin_prepare_release command` is useful to
format the changelogs and create the tags, but will
fail if no remote repository is configured for the current
branch.

Since I could not see a reaason for this limitation, I propose
the` --no-push` option, that will do everything locally and not
push anything.

This was done for my own convenience. I submit it as a pull
request just in case it would be useful to someone else.