core: Fixes logging in SshSoftFencingCommand

1) Reverts logging command output in SshSoftFencingCommand erroneously
   committed in 52716e7498675d71654787eec21956fd30d38edf.
2) Adds logging of command error output

Change-Id: I6ea12003bd44f6500e3878176e98debd102da056
Signed-off-by: Martin Perina <mperina@redhat.com>