Fix: fencing,libfencing: use level tag consistently in internal XML

12ca8bb and a4f67f5 incompletely coordinated the tag name that is set and
expected for managing topology levels. This revision gets rid of the
F_STONITH_LEVEL ("st_level") tag completely, and uses the XML_TAG_FENCING_LEVEL
("fencing-level") tag consistently in internal XML as well as the CIB.