Make the Pipeline inmutable

This patch makes pipeline inmutable by removing the append method.
the pipeline metadata now returns a pipeline instance.

Change-Id: I9b90933a83ca1d423d3718f438d3717c39ef78d4
Closes-Bug: #1433158

zaqar/common/pipeline.py

@@ -45,9 +45,6 @@ class Pipeline(object):
     def __init__(self, pipeline=None):
         self._pipeline = pipeline and list(pipeline) or []
 
-    def append(self, stage):
-        self._pipeline.append(stage)
-
     @decorators.memoized_getattr
     def __getattr__(self, name):
         with self.consumer_for(name) as consumer:

zaqar/storage/pipeline.py

@@ -78,7 +78,7 @@ def _get_storage_pipeline(resource_name, conf):
                         {'stage': ns, 'ex': str(exc)})
             continue
 
-    return common.Pipeline(pipeline)
+    return pipeline
 
 
 class DataDriver(base.DataDriverBase):
@@ -109,22 +109,22 @@ class DataDriver(base.DataDriverBase):
     def queue_controller(self):
         stages = _get_storage_pipeline('queue', self.conf)
         stages.append(self._storage.queue_controller)
-        return stages
+        return common.Pipeline(stages)
 
     @decorators.lazy_property(write=False)
     def message_controller(self):
         stages = _get_storage_pipeline('message', self.conf)
         stages.append(self._storage.message_controller)
-        return stages
+        return common.Pipeline(stages)
 
     @decorators.lazy_property(write=False)
     def claim_controller(self):
         stages = _get_storage_pipeline('claim', self.conf)
         stages.append(self._storage.claim_controller)
-        return stages
+        return common.Pipeline(stages)
 
     @decorators.lazy_property(write=False)
     def subscription_controller(self):
         stages = _get_storage_pipeline('subscription', self.conf)
         stages.append(self._storage.subscription_controller)
-        return stages
+        return common.Pipeline(stages)