Fix cross-namespace cli operations when a context is set

[andreaTP]

What changes were proposed in this pull request?

Fix #961
Setting a default namespace in the current context will break the kubectl-cloudflow functionalities.

E.g.:

$ kubectl config set-context --current --namespace="non-existing"
$ kubectl cloudflow list
Error: No Cloudflow operators detected in the cluster. Exiting

Current workaround:

$ kubectl config set-context --current --namespace=""

Why are the changes needed?

Enable the usage of kubectl-cloudflow even when a namespace is defined in the current context.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Running the integration tests on GKE after setting a namespace in the context.
Running most of the operations (e.g. deploy/undeploy/configure/configuration/list/status) on a simple application on an OpenShift cluster.

cc. @thomasschoeftner @OBenner @michael-read

[RayRoestenburg]

LGTM!