versioning: misc cleanups

highlights:
 - NetConf struct finally includes cniVersion field
 - improve test coverage of current version report behavior
 - godoc a few key functions
 - allow tests to control version list reported by no-op plugin

types/types.go

@@ -57,6 +57,8 @@ func (n *IPNet) UnmarshalJSON(data []byte) error {
 
 // NetConf describes a network.
 type NetConf struct {
+	CNIVersion string `json:"cniVersion,omitempty"`
+
 	Name string `json:"name,omitempty"`
 	Type string `json:"type,omitempty"`
 	IPAM struct {