Nailgun extensions in stevedore

The change introduces Nailgun extensions which use stevedore to
plug into nailgun. Stevedore enables (not only) Fuel developers
to write extensions and develop them independently in separate
repos.

Functions for loading and retriving extensions classes was
moved to new file `manager.py`.

Change-Id: I59015a28f460b1e45312b1c003aadec3cc396ad5
Implements: blueprint stevedore-extensions-discovery

cluster_upgrade/extension.py

@@ -24,6 +24,7 @@ from . import handlers
 class ClusterUpgradeExtension(extensions.BaseExtension):
     name = 'cluster_upgrade'
     version = '0.0.1'
+    description = "Cluster Upgrade Extension"
 
     urls = [
         {'uri': r'/clusters/(?P<cluster_id>\d+)/upgrade/clone/?$',