[bfss] convert root BFSS directory to str in constructor (closes #2137793)
otherwise you can get crashed with AssertionError: Binary objects must use raw strings, not <type 'unicode'>
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared