Refine multus config file (masterplugin and always_use_default)

This fix removes 'masterplugin' and 'always_use_default' from
multus config file. default network, defined in
/etc/cni/net.d/*conf should be master and this network should be
used as default from network plumbing working group draft.

types/types.go

@@ -29,7 +29,6 @@ type NetConf struct {
 	CNIDir     string                   `json:"cniDir"`
 	Delegates  []map[string]interface{} `json:"delegates"`
 	Kubeconfig string                   `json:"kubeconfig"`
-	UseDefault bool                     `json:"always_use_default"`
 }
 
 type Network struct {