python: Support resource clearing
    
    Add a clear kwarg to sanlock.write_resource(), using SANLK_WRITE_CLEAR
    to clear the resource.
    
    To clear a resource, you can do now:
    
        sanlock.write_resource("lockspace", "resource", [("/path/to/resource", 0)],
                               clear=True)
    
    Subsequent sanlock.read_resource() calls will return the regular error
    when a resource is not found.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>