Revise Antrea NetworkPolicies short names

[abhiraut]

Antrea NetworkPolicy CRDs can be retrieved using kubectl commands using either full name or short name. However, the short name netpol for Antrea NetworkPolicy collides with that of K8s NetworkPolicy, in which case, the K8s NetworPolicies are returned. In order to retrieve Antrea NetworkPolicy, users must append the short name with the group name. eg
kubectl get networkpolicies.security
The above is a little counterintuitive and is different than the short name used for Antrea ClusterNetworkPolicy.

Based on the discussion in the community meeting, following is one of the proposal to make the short names for policies uniform.

Antrea ClusterNetworkPolicy
Current short name: kubectl get cnp
Proposed short name1: kubectl get acnp 
Proposed short name2: keep same

Antrea NetworkPolicy
Current short name: kubectl get netpol.security
Proposed short name: kubectl get anp

[abhiraut]

/cc @antoninbas @tnqn @jianjuns @salv-orlando

[antoninbas]

I think standardizing the names to "antreaclusternetworkpolicies" / "acnp" and "antreanetworkpolicies" / "anp" would be good and may help avoid any conflicts in the future as the upstream NetworkPolicy APIs evolve. If changing the full resource names is too disruptive, adding"acnp" and "anp" as short names (we can preserve the existing ones for now) is probably a good start. When we upgrade the API version, we can revisit full names. If we decided to switch to "antreaclusternetworkpolicies" / "antreanetworkpolicies", we would need to ensure consistency with the new metrics API (https://github.com/vmware-tanzu/antrea/pull/1172/files#r487954032).

[abhiraut]

Fixed by #1291