Parses a port forwarding statement in the form used by this state:

    from_port:to_port:protocol[:destination]

    and returns a ForwardingMapping object

def _parse_forward(mapping):
    '''
    Parses a port forwarding statement in the form used by this state:

    from_port:to_port:protocol[:destination]

    and returns a ForwardingMapping object
    '''
    if len(mapping.split(':')) > 3:
        (srcport, destport, protocol, destaddr) = mapping.split(':')
    else:
        (srcport, destport, protocol) = mapping.split(':')
        destaddr = ''
    return ForwardingMapping(srcport, destport, protocol, destaddr)