New script utils/changelog

This scrip can be used to generate the changelog text for a new
ansible-freeipa release on github.

    usage: Usage: changelog [options] [<new version>]

    optional arguments:
      -h, --help  show this help message and exit
      --tag TAG   git tag

If the script is used without a givn tag, it will show all the changes since
the last tag. If a tag (this can be a also a commit) is given, then all
changes since this commit are shown.