K8s: keeping namespace label set in cilium NP
K8s: keeping namespace label set in cilium NP

Cilium Network Policy only enforces policies on the namespace where it's
stored. This is achieved by adding a special label on the
EndpointSelector, on the FromEndpoints and on the FromRequires with the
k8s namespace where the policy is stored.

Since users are allowed to specify that special label on the
FromEndpoints and / or FromRequires, we should keep the value's label
unchanged.

The special label in the EndpointSelector is only possible to be
overwritten with the metadata's namespace. If the user specifies it
manually, by inserting the special label, a warning is printed in cilium
logs and the label is replaced with the namespace where the policy is
stored.

For example, the following CNP
```
metadata:
  name: "l7-stresstest"
  namespace: "development"
spec:
  endpointSelector:
    matchLabels:
      id: server
  ingress:
  - fromEndpoints:
    - matchLabels:
        id: client
        "k8s:io.kubernetes.pod.namespace": qa
```

Will be translated into:
```
"endpointSelector": {
  "matchLabels": {
    "any:id": "server",
    "k8s:io.kubernetes.pod.namespace": "development"
  }
},
"ingress": [
  {
    "fromEndpoints": [
      {
        "matchLabels": {
          "any:id": "client",
          "k8s:io.kubernetes.pod.namespace": "qa"
        }
      }
    ],
```

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

gumpt/cilium,scanf/cilium,mskarbek/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,scanf/cilium,scanf/cilium,eloycoto/cilium,gumpt/cilium,cilium/cilium,tklauser/cilium,gumpt/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,mskarbek/cilium,michi-covalent/cilium,tklauser/cilium,eloycoto/cilium,michi-covalent/cilium,mskarbek/cilium,eloycoto/cilium,tklauser/cilium,tgraf/cilium,eloycoto/cilium,gumpt/cilium,cilium-team/cilium,tgraf/cilium,cilium-team/cilium,tgraf/cilium,cilium/cilium,mskarbek/cilium,michi-covalent/cilium,michi-covalent/cilium,mskarbek/cilium,tklauser/cilium,gumpt/cilium,scanf/cilium,eloycoto/cilium,tgraf/cilium,gumpt/cilium,mskarbek/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,eloycoto/cilium,gumpt/cilium,scanf/cilium