SelfIP removal can conflict with default route

[notandy]

sync_l2_selfips_flow adds and removes selfips if, for example, a LB has been removed from another subnet.
But if this subnet is used as a default GW, this will cause an exception because F5 forbids to remove a selfip in the same subnet as the default GW.

sync_l2_selfips_flow should also update the default route to ensure this error doesn't happens.

[BenjaminLudwigSAP]

We assume that this didn't lead to bigger issues so far because it is reconciled with the next full sync, which then changes the default route to the remaining subnet, allowing the SelfIP of the now LB-less subnet to be deleted.