Currently test `DvrRouterNamespace` in Tobiko is only looking for L3 agents on compute nodes: https://opendev.org/x/tobiko/src/commit/3297a7628cc4ce115cfa8c2aa0951dfb741bdb0e/tobiko/tests/scenario/neutron/test_router.py#L241 but such agent can also run on the controller and/or networker nodes. 

Test is currently failing if DVR router's snat is scheduled to be on controller node as test is expecting `snat-` name to be on the compute node where L3 agent_mode is set to `dvr_snat`.

We should probably change Tobiko framework in the way that it would:

• check L3 agent mode on all nodes,
• look for the namespace on the node(s) where router is actually scheduled - this could be first checked with Neutron API.