core: Adds SSH Soft Fencing capability

Adds SshSoftFencingCommand which provides ability to restart VDSM
on the host using SSH connection. SshSoftFencingCommand is executed
prior to standard fencing process. If the VDSM restart doesn't help,
standard fencing process will be executed. SshSoftFencingCommand is also
executed for host without power management configured (on these hosts
standard fencing cannot be executed).

Change-Id: I8002b6ac00a1e2e543b5cc8d1affdd42b994d5f7
Bug-Url: https://bugzilla.redhat.com/967328
Signed-off-by: Martin Perina <mperina@redhat.com>