Centralize plugin loaders in watcher/applier

Refactor all plugin loaders in watcher/applier into
watcher/applier/loading/default.py.

Change-Id: I0f0a87d4f72ead45d34aca1b14219fd2ede42a6f
Closes-Bug: #1591095

watcher/applier/loading/default.py

@@ -0,0 +1,29 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import unicode_literals
+
+
+from watcher.common.loader import default
+
+
+class DefaultWorkFlowEngineLoader(default.DefaultLoader):
+    def __init__(self):
+        super(DefaultWorkFlowEngineLoader, self).__init__(
+            namespace='watcher_workflow_engines')
+
+
+class DefaultActionLoader(default.DefaultLoader):
+    def __init__(self):
+        super(DefaultActionLoader, self).__init__(
+            namespace='watcher_actions')