Determine default IPv4 route device only when needed

Sometimes instances don't have an IPv4 default route, so only check for
it when we actually need it. In a followup patch we could extend the
code to check for an IPv6 default route instead or in addition.

Related-Bug: 1902002
Change-Id: Ie6cd241721f6b1f8e030960921a696939b2dab10
1 file changed