Determine default IPv4 route device only when needed
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

nawawi/openstack,nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack