[v4.1.z] Fix underscores for non-suffix plurals

[cben]

This PR is not onto master! I've cut v4.1.z branch from v4.1.0 tag and want to release this as 4.1.1, then merge this into master before doing anything else on master.

Fixes #376, a regression in 4.1.0. Partially undoes 783c58b from #366.

Restores underscores in following method names (all of these prefixed with get_, create_ etc):

• network_policy network_policies
• pod_security_policy pod_security_policies
• egress_network_policy egress_network_policies
• cluster_policy cluster_policies
• network_policy network_policies

These are the known examples from k8s and openshift (exact differences 4.0.0 -> 4.1.0 -> this PR can be seen in this gist).
In principle may similarly affect other plurals in custom resources, eg. (fictional) big_datum / big_data.

(duh, if only I carefully read the comments before that refactoring, they specifically showed networkpolicies plural and matching only on NetworkP prefix...)

WIP because need changelog & I want to add another test.

[cben]

@masayag @grosser please review.

Moti, I stole some of your tests ;-). You'll have to rebase but you'd need anyway to adopt to this partial prefix matching.

[masayag]

Looks good. About rebase - shouldn't #373 be merged to master first ? In that case, no need to rebase.

Unless you intend it to for 4.1.z only as well.