Do not use deperecated MetaData.bind

Sqlaclhemy metadata bind is deperecated from version 1.4 and will
be removed in SQLAlchemy 2.0 [1]. It can be easily replaced with
metadata reflect which works for SQLAlchemy 2.0.

Also current metadata.bind does not result in fetching all tables from
the database, so metadata.tables is an empty mapping, which leads
to db_sync failures.

[1] https://docs.sqlalchemy.org/en/14/core/metadata.html#sqlalchemy.schema.MetaData.bind
[2] https://docs.sqlalchemy.org/en/14/core/reflection.html#reflecting-all-tables-at-once

Change-Id: I2b6f5f7e58567759b631baad87861d3a135f60fa

senlin/db/sqlalchemy/alembic/legacy_utils.py

@@ -23,7 +23,7 @@ LOG = logging.getLogger(__name__)
 
 def is_migration_needed(equivalent_revision):
     metadata = sa.MetaData()
-    metadata.bind = op.get_bind()
+    metadata.reflect(bind=op.get_bind())
 
     if 'migrate_version' not in metadata.tables.keys():
         return True