Fix lb-vip does not get route to default gw

Previously when creating a lb-vip it would be created without
a default gw. This patch fixes that and adds unit tests to check
that route add is called if the subnet has a gateway_ip.

Fixes bug 1162626

Change-Id: I155749fa6d9c843fca87a73f3cf85720aac26cfa
(cherry picked from commit 37b41833bf4f9500ee6be53d15298d0f4b964ea3)