kdump-lib: add common variables and function for fence kdump
    
    Add following common variables and function:
    
    $FENCE_KDUMP_CONIFG: configuration file /etc/sysconfig/fence_kdump
    $FENCE_KDUMP_NODES: configuration file /etc/fence_kdump_nodes
    $FENCE_KDUMP_SEND: executable /usr/libexec/fence_kdump_send
    is_fence_kdump(): used to determine if the system is in a cluster and
    configured with fence_kdump.
    
    Signed-off-by: WANG Chao <chaowang@redhat.com>
    Tested-by: Zhi Zou <zzou@redhat.com>
    Tested-by: Marek Grac <mgrac@redhat.com>
    Acked-by: Vivek Goyal <vgoyal@redhat.com>