Support deploying to multiple kubernetes contexts

[balopat]

This came up as an idea in #104.

In a multi-artifact project, skaffold would be able to deploy each artifact to a different namespace/context.

If you think your use case would need this functionality, please get in touch and describe it for us here!

[derekperkins]

We run in a multi-cloud environment, so it's pretty annoying to have to remember which kubectl context you're in before deploying. It's a pretty easy way to accidentally deploy to the wrong region. I'd love to make skaffold cluster aware somehow.

[haf]

I'd like to deploy all the monitoring locally in ns:monitoring and the app in app, but now:

[tejal29]

related to #2325.

[haf]

I can also chime in that deploying everything with kustomize rather than kubectl does it right when multiple namespaces are involved, so IMO that could be documented and this issue closed.

[nkubala]

I think kustomize opens up enough flexibility to support use cases described here, and I don't think we're going to have time to implement something deeper in skaffold, so I'll close this issue. if you still feel that skaffold needs to support this more fully, please give us some details on your use case and feel free to reopen!

[revero-doug]

skaffold/v3 now uses kustomize only for resource generation and applies using kubectl, so there is seemingly no longer a workaround here . this feels especially broken when requiring another skaffold config with a different defaultNamespace

edit: you can still work around this using kustomize (specifically by setting /namespace in kustomization.yaml), but /deploy/kubectl/defaultNamespace must never be set in any referenced config if any manifest across all configs explicitly sets a conflicting namespace. IMO with these new semantics of /deploy/kubectl/defaultNamespace, it should be renamed /deploy/kubectl/namespace because it doesn't behave like setting a default, but rather like setting the -n argument on a kubectl invocation

It's also worth noting that kustomize itself doesn't implement "defaultNamespace" behavior by default, but the NamespaceTransformer may be customized to switch from always-write to write-if-unset semantics.

[msonnleitner]

We use GCP's Multi-Cluster Ingress to load-balance traffic to multiple GKE clusters. It would be nice if Skaffold could deploy the same application to multiple clusters

[aaron-prindle]

@gsquared94 to split this out into multiple issues - multiple contexts vs multiple namespaces