By default pods in Kubernetes can connect to each other. However in
Qinling, each pod should be independent acting as a worker of a function
or running an execution of a function. Disabling the inter-pods traffic
in the namespace used by Qinling would ensure the isolation of each pod.
This commit leverages the NetworkPolicy in Kubernetes to isolate the
pods. So a network solution which supports NetworkPolicy (for example,
cailco) for Kubernetes must be used or there will be no effect.