Fix path that housekeeping.py think the __hk_* utitlities are in.

The housekeeping.py script tries to execute the __hk_{copy,delete,move}
command in a non-existent '$CYLC_DIR/util' directory.
The correct location is '$CYLC_DIR/bin'.
Merge pull request #736 from areinecke/housekeeping

Fix path that housekeeping.py think the __hk_* utitlities are in.