rose: use Python to do "readlink -f" if necessary.

"readlink -f" is GNU extension.
rose: remove unnecessary logic.

No need to cannonize a path twice.
Merge pull request #754 from matthewrmshin/readlink-f

rose: use Python to do "readlink -f" if necessary.
Merge branch 'master' of github.com:metomi/rose into gh-pages