router-advert: T2618: fix for wrongly converted "autonomous-flag"
router-advert: T2618: fix for wrongly converted "autonomous-flag"

Boolean nodes have been removed b/c a boolean value makes less sense. The same
behavior can be achieved with the presence or absence of a node. Unfortunately
the 'autonomous-flag' node has been a negated one which did not migrate
it to 'no-autonomous-flag' if it was set to false previously.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x