Use a writer context for the online alembic migrations

``DBConnection`` class does not retrieve the URL string
correctly and cannot connect to the database. This patch
replaces it with default writer context that any database
operation transaction uses in Neutron.

Closes-Bug: #2015847

Change-Id: I60523690bc170ec4ba81312ada0f2d83542cae67

neutron/db/migration/alembic_migrations/env.py

@@ -13,14 +13,16 @@
 #    under the License.
 
 from alembic import context
+from neutron_lib import context as n_context
+from neutron_lib.db import api as db_api
 from neutron_lib.db import model_base
 from oslo_config import cfg
+from oslo_db import options as db_options
 import sqlalchemy as sa
 from sqlalchemy import event  # noqa
 
 from neutron.db.migration.alembic_migrations import external
 from neutron.db.migration import autogen
-from neutron.db.migration.connection import DBConnection
 from neutron.db.migration.models import head  # noqa
 
 try:
@@ -53,6 +55,13 @@ def set_mysql_engine():
                     model_base.BASEV2.__table_args__['mysql_engine'])
 
 
+def setup_conf():
+    conf = cfg.CONF
+    conf.register_opts(db_options.database_opts, 'database')
+    conf.set_override('connection', neutron_config.database.connection,
+                      group='database')
+
+
 def include_object(object_, name, type_, reflected, compare_to):
     if type_ == 'table' and name in external.TABLES:
         return False
@@ -101,11 +110,12 @@ def run_migrations_online():
     and associate a connection with the context.
 
     """
+    setup_conf()
     set_mysql_engine()
-    connection = config.attributes.get('connection')
-    with DBConnection(neutron_config.database.connection, connection) as conn:
+    admin_ctx = n_context.get_admin_context()
+    with db_api.CONTEXT_WRITER.using(admin_ctx) as session:
         context.configure(
-            connection=conn,
+            connection=session.connection(),
             target_metadata=target_metadata,
             include_object=include_object,
             process_revision_directives=autogen.process_revision_directives