use mangle_ppa_shortcut consistently.

this fixes a bug/issue in the python2 path with
  python -m softwareproperties.ppa ppa:~smoser/ppa

The issue was that launchpad would return a 301 and pycurl would
not follow the redirect.
  https://launchpad.net/api/devel/~smoser/+archive/ppa
would get a 301 to
  https://launchpad.net/api/devel/~smoser/+archive/ubuntu/ppa