[AIRFLOW-2574] Cope with '%' in SQLA DSN when running migrations (#3787)

Alembic uses a ConfigParser like Airflow does, and "%% is a special
value in there, so we need to escape it. As per the Alembic docs:

> Note that this value is passed to ConfigParser.set, which supports
> variable interpolation using pyformat (e.g. `%(some_value)s`). A raw
> percent sign not part of an interpolation symbol must therefore be
> escaped, e.g. `%%`

Author: ashb

airflow/utils/db.py

@@ -341,8 +341,8 @@ def upgradedb():
     package_dir = os.path.normpath(os.path.join(current_dir, '..'))
     directory = os.path.join(package_dir, 'migrations')
     config = Config(os.path.join(package_dir, 'alembic.ini'))
-    config.set_main_option('script_location', directory)
-    config.set_main_option('sqlalchemy.url', settings.SQL_ALCHEMY_CONN)
+    config.set_main_option('script_location', directory.replace('%', '%%'))
+    config.set_main_option('sqlalchemy.url', settings.SQL_ALCHEMY_CONN.replace('%', '%%'))
     command.upgrade(config, 'heads')