Fix warnings when the VIM loads plugins

stevedore was treating the string as a list of characters
The code worked, but was littering the logs with warnings

Change-Id: I40f401aab4927e284cb06752977858c4bcecedf0

nfv/centos/build_srpm.data

@@ -1 +1 @@
-TIS_PATCH_VER=73
+TIS_PATCH_VER=74

nfv/nfv-vim/nfv_vim/nfvi/_nfvi_plugin.py

@@ -23,7 +23,7 @@ class NFVIPlugin(object):
         self._scheduler = scheduler
 
         plugins = stevedore.named.NamedExtensionManager(namespace,
-                                                        plugin_type,
+                                                        [plugin_type, ],
                                                         invoke_on_load=True,
                                                         invoke_args=(),
                                                         invoke_kwds={})