echo-external-node: Check if NodeWithoutCilium is enabled

[michi-covalent]

Create echo-external-node deployment if and only if there is a node with "cilium.io/no-schedule=true" label. Otherwise Kubernetes cannot schedule this deployment.

Fixes: d541be5 ("connectivity: add echo-external deployment")
Fixes: cilium/cilium#27009

[joestringer]

Does this automatically disable the tests that rely on this deployment?

[michi-covalent]

Does this automatically disable the tests that rely on this deployment?

yeah, it's a bit implicit but it checks if ExternalEchoPods is empty:

• cilium-cli/connectivity/tests/egressgateway.go

  Line 237 in 951eb91

  for _, externalEcho := range ct.ExternalEchoPods() {

• cilium-cli/connectivity/tests/egressgateway.go

  Line 253 in 951eb91

  for _, client := range ct.ExternalEchoPods() {

• cilium-cli/connectivity/tests/egressgateway.go

  Line 278 in 951eb91

  for _, client := range ct.ExternalEchoPods() {

• cilium-cli/connectivity/tests/egressgateway.go

  Line 369 in 951eb91

  for _, externalEcho := range ct.ExternalEchoPods() {

so these tests won't run if there is no echo-external-node pod.

[nebril]

Looks good, one quality of life comment left inline.