Rename FENCE_KDUMP_CONFIG to FENCE_KDUMP_CONFIG_FILE
    
    Renames FENCE_KDUMP_CONFIG variable to FENCE_KDUMP_CONFIG_FILE to
    distinguish it from values read from fence_kdump_args option in
    kdump.conf (introduced in following patches).
    
    Bug-Url: https://bugzilla.redhat.com/1078134
    Signed-off-by: Martin Perina <mperina@redhat.com>
    Acked-by: Vivek Goyal <vgoyal@redhat.com>