Cleanup some minor issues in PluginLoader:

* class_only was a keyword arg of get() and all() that was mistakenly
  passed on to Plugins.  Be sure to strip it from the keyword args
  before instantiating Plugins.  (Reworked API probably should either
  not instantiate Plugins or take the args for the Plugin as a separate
  list and a dict.)
* Checking required base_classes was only done in get() and only if
  class_only was False (ie: that Plugin was instantiated).  This meant
  that different plugins could be found depending on whether the call
  was to .get() or to all() and whether it was for classes or instances.
  Fixed so that required base_classes are always checked.

lib/ansible/plugins/__init__.py

@@ -316,6 +316,7 @@ class PluginLoader:
     def get(self, name, *args, **kwargs):
         ''' instantiates a plugin of the given name using arguments '''
 
+        class_only = kwargs.pop('class_only', False)
         if name in self.aliases:
             name = self.aliases[name]
         path = self.find_plugin(name)
@@ -325,23 +326,28 @@ class PluginLoader:
         if path not in self._module_cache:
             self._module_cache[path] = self._load_module_source('.'.join([self.package, name]), path)
 
-        if kwargs.get('class_only', False):
         obj = getattr(self._module_cache[path], self.class_name)
-        else:
-            obj = getattr(self._module_cache[path], self.class_name)(*args, **kwargs)
         if self.base_class:
             # The import path is hardcoded and should be the right place,
             # so we are not expecting an ImportError.
             module = __import__(self.package, fromlist=[self.base_class])
             # Check whether this obj has the required base class.
-                if not issubclass(obj.__class__, getattr(module, self.base_class, None)):
+            try:
+                plugin_class = getattr(module, self.base_class)
+            except AttributeError:
                 return None
+            if not issubclass(obj, plugin_class):
+                return None
+
+        if not class_only:
+            obj = obj(*args, **kwargs)
 
         return obj
 
     def all(self, *args, **kwargs):
         ''' instantiates all plugins with the same arguments '''
 
+        class_only = kwargs.pop('class_only', False)
         for i in self._get_paths():
             matches = glob.glob(os.path.join(i, "*.py"))
             matches.sort()
@@ -353,14 +359,22 @@ class PluginLoader:
                 if path not in self._module_cache:
                     self._module_cache[path] = self._load_module_source(name, path)
 
-                if kwargs.get('class_only', False):
                 obj = getattr(self._module_cache[path], self.class_name)
-                else:
-                    obj = getattr(self._module_cache[path], self.class_name)(*args, **kwargs)
-
-                    if self.base_class and self.base_class not in [base.__name__ for base in obj.__class__.__bases__]:
+                if self.base_class:
+                    # The import path is hardcoded and should be the right place,
+                    # so we are not expecting an ImportError.
+                    module = __import__(self.package, fromlist=[self.base_class])
+                    # Check whether this obj has the required base class.
+                    try:
+                        plugin_class = getattr(module, self.base_class)
+                    except AttributeError:
+                        continue
+                    if not issubclass(obj, plugin_class):
                         continue
 
+                if not class_only:
+                    obj = obj(*args, **kwargs)
+
                 # set extra info on the module, in case we want it later
                 setattr(obj, '_original_path', path)
                 yield obj