Make backends into drivers, loadable as entry points

Added base.py, which has classes for backends to derive from. This is
the stevedore suggested way of writing drivers.
stevedore.readthedocs.org/en/latest/tutorial/creating_plugins.html

Added entrypoints to setup.cfg

implements blueprint backend-abstraction
Change-Id: Ib53bd1ec930f15480e33b9105033e9999f96b842

entropy/backends/base.py

@@ -0,0 +1,39 @@
+# Copyright (C) 2013 Yahoo! Inc. All Rights Reserved.
+#
+# 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.
+
+import abc
+
+import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class Backend(object):
+    """Base class for persistence backends."""
+
+    def __init__(self, conf):
+        if not conf:
+            conf = {}
+        if not isinstance(conf, dict):
+            raise TypeError("Configuration dictionary expected not: %s"
+                            % type(conf))
+        self._conf = conf
+
+    @abc.abstractmethod
+    def open(self):
+        pass
+
+    @abc.abstractmethod
+    def close(self):
+        """Closes any resources this backend has open."""
+        pass

entropy/backends/file_backend.py

@@ -11,3 +11,13 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
+import os
+
+from entropy.backends import base
+
+
+class FileBackend(base.Backend):
+    """A directory based backend."""
+    def __init__(self, conf):
+        super(FileBackend, self).__init(conf)
+        self.path = os.path.abspath(conf['path'])

setup.cfg

@@ -6,7 +6,7 @@ description-file =
     README.md
 author = Entropy devs
 author-email = praneshpg@gmail.com
-home-page = http://entropy.readthedocs.org/
+home-page = https://wiki.openstack.org/wiki/Entropy
 classifier =
     Development Status :: 4 - Beta
     Environment :: Console
@@ -26,6 +26,10 @@ classifier =
 console_scripts =
     entropy = entropy.__main__:main
 
+entropy.backend =
+    file = entropy.backends.file_backend:FileBackend
+    db = entropy.backends.file_backend:DbBackend
+
 [global]
 setup-hooks =
     pbr.hooks.setup_hook