Fix: crmd: Prevent manual fencing confirmations from attemping to create node entries for unknown nodes

An user used the following command:
stonith_admin -C=qaxen8

He didn't realize that the equal sign was not needed with the "-C".  But the cluster aborted transition and attempted to create node entry for "=qaxen8", and that resulted in an additional  '<node_state uname="=qaxen8" />' in cib.
(corosync-1.4.6 plugin-based)

It seems we should not create node entry for an unknown node when getting a fencing confirmation. Does the fix make sense? Thanks!