Use stevedore directive to document plugins

Add directives to the documentation to bring in the docstrings from the
plugins.

Change-Id: I2afe1d1e4a9abfa1d3cdf85bc4008ff8f772123c

doc/source/conf.py

@@ -23,7 +23,8 @@ extensions = [
     'sphinx.ext.autodoc',
     #'sphinx.ext.intersphinx',
     'oslo_config.sphinxext',
-    'oslosphinx'
+    'oslosphinx',
+    'stevedore.sphinxext'
 ]
 
 # autodoc generation is a bit aggressive and a nuisance when doing heavy

doc/source/usage.rst

@@ -136,3 +136,11 @@ DB API backend support
     # DB-API method
     def do_something(somethind_id):
         return IMPL.do_something(somethind_id)
+
+DB migration extensions
+=======================
+
+Available extensions for :mod:`oslo_db.migration`.
+
+.. list-plugins:: oslo.db.migration
+    :detailed:

oslo_db/sqlalchemy/migration_cli/ext_alembic.py

@@ -21,6 +21,13 @@ from oslo_db.sqlalchemy.migration_cli import ext_base
 
 
 class AlembicExtension(ext_base.MigrationExtensionBase):
+    """Extension to provide alembic features.
+
+    :param engine: SQLAlchemy engine instance for a given database
+    :type engine: sqlalchemy.engine.Engine
+    :param migration_config: Stores specific configuration for migrations
+    :type migration_config: dict
+    """
 
     order = 2
 
@@ -29,13 +36,6 @@ class AlembicExtension(ext_base.MigrationExtensionBase):
         return os.path.exists(self.alembic_ini_path)
 
     def __init__(self, engine, migration_config):
-        """Extension to provide alembic features.
-
-        :param engine: SQLAlchemy engine instance for a given database
-        :type engine: sqlalchemy.engine.Engine
-        :param migration_config: Stores specific configuration for migrations
-        :type migration_config: dict
-        """
         self.alembic_ini_path = migration_config.get('alembic_ini_path', '')
         self.config = alembic_config.Config(self.alembic_ini_path)
         # TODO(viktors): Remove this, when we will use Alembic 0.7.5 or