HA routers are using keepalived and needs to have virtual_router_id
configured. As routers which belongs to same tenant are using same
ha network, those values have to be different for each router.
Before this patch this value was always taken as first available value
from available_vr_ids range.
In some (rare) cases, when more than one router is created in parallel
for same tenant it may happen that those routers would have same vr_id
choosen so keepalived would treat them as single application and only
one router would be ACTIVE on one of L3 agents.
This patch changes this behaviour that now random value from available
vr_ids will be chosen instead of taking first value always.
That should mittigate this rare race condition that it will be (almost)
not noticable for users.
However, proper fix should be probably done as some additional
constraint in database layer. But such solution wouldn't be possible to
backport to stable branches so I decided to propose this easy patch
first.
Change-Id: Idb0ed744e54976dca23593fb2d7317bf77442e65
Related-Bug: #1823314
a8d0f557d5