Set default mesh gateway only if no gateway is specified

[stefanprodan]

Fix: #140
Note: breaking change

[stefanprodan]

@SpreetailHurley can you please test this PR and confirm that canary routing works with the ingress gateway? Thanks

Test image: quay.io/weaveworks/flagger:default-mesh-88ad457

[stefanprodan]

I don't know if it's relevant to your use case but the ClusterIP address is always added to the hosts list.

[SpreetailHurley]

@stefanprodan Looks perfect on the surface; I'll run some more testing and confirm in this thread.

Thanks!

[SpreetailHurley]

All testing completed successfully:

Specify gateway list not including 'mesh'

• VirtualService gateways includes listed gateways; does not include mesh
• validate that VirtualService rewrites apply to Internet traffic
• validate that VirtualService rewrites do not apply to mesh traffic

Specify gateway list including but not exclusively 'mesh'

• VirtualService gateways includes all listed gateways; does include mesh
• validate that VirtualService rewrites apply to Internet traffic
• validate that VirtualService rewrites apply to mesh traffic

Specify no gateway list

• VirtualService gateways includes only mesh
• validate that VirtualService rewrites apply to Internet traffic
• validate that VirtualService rewrites apply to mesh traffic

[SpreetailHurley]

It should also be noted that case 1 (specify a gateway list that does not include 'mesh') will necessitate that connections inbound for testing either:

1. target the canary service (cmd: "hey -z 1m -q 10 -c 2 -H 'Cookie: user=test' http://my-service-canary/my-path")
2. path out to re-traverse the gateway
3. are executed externally (e.g. a dedicated load generator or testing framework that exists outside of the cluster)

This is expected behavior because connections from inside the cluster only traverse the 'mesh' gateway and so would not be subject to routing rules nor to targeting the canary via regex or other matches in the VirtualService.

[stefanprodan]

@SpreetailHurley thanks a lot for all the testing 🥇