core: Adds config of SSH Soft Fencing command

Adds SshSoftFencingCommand configuration value, that contains command
to execute on host using SSH when host VDSM doesn't respond. By default
it contains command to restart VDSM.

Change-Id: I9f96cbe0c5fdc33f8f512c36d1cd85349852f2da
Bug-Url: https://bugzilla.redhat.com/967328
Signed-off-by: Martin Perina <mperina@redhat.com>