core: Add config value to enable to skip fencing if SD is active

Adds config value to enable feature to skip fencing of host when it's
connected to at least one storage domain.

Change-Id: If9f1e480a5cedb7988eba37701c3db7b3950c010
Bug-Url: https://bugzilla.redhat.com/1090799
Signed-off-by: Martin Perina <mperina@redhat.com>