Istio sidecar injection for all user namespaces

[mdemirhan]

Expected Behavior

Istio sidecar injection should be enabled not only for default namespace but for all other namespaces that revisions can get created in.

Actual Behavior

Only default namespace is enabled for automatic sidecar injection.

[ZhiminXiang]

I want to confirm that istio sidecar injections should NOT be enabled for Elafros system namespaces such as ela-system, build-system, etc. Is my understanding correct?

[ZhiminXiang]

Per our offline discussion, we may want to inject Istio sidecars into pods of autoscaler and activator that are under namespace ela-system. For other pods under ela-system, we do not need to inject Istio sidecars.

An alternative thought is to move autoscaler and activator into another namespace.

[evankanderson]

/kind feature

/assign @tcnghia

I think this should be handled by istio/istio#6476 (comment) ?

[tcnghia]

dup of #771