Allow serverless outbound traffic to external services by default

Description

We are currently using Knative version 0.6 and it comes with Istio version 1.0.7.

This Istio version does not allow egress to external services by default. It is necessary to whitelist all the external services you want to access from within the mesh, one by one, see https://istio.io/docs/tasks/traffic-management/egress/egress-control/#controlled-access-to-external-services.

Example:

kubectl apply -f - <<EOF
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: gitlab-https-ext
spec:
  hosts:
  - gitlab.com
  ports:
  - number: 443
    name: https
    protocol: HTTPS
  resolution: DNS
  location: MESH_EXTERNAL
EOF

Proposal

Update Istio / configuration to use mode: ALLOW_ANY to enable egress to any external service by default.

/cc @danielgruesso @DylanGriffith