fix(contrib): use absolute path to user-data

if you run the rackspace provision script outside of contrib/rackspace,
it will fail because it will look for a user-data file relative to
your current working directory. Using $CONTRIB_DIR to determine where
the user data is located fixes this.
Merge pull request #2641 from bacongobbler/rackspace-contrib-dir

fix(contrib): use absolute path to user-data