title: SqlAlchemy alembic migration files dont use the connection settings in env.py?
body: <p>I have a migration that works on a generic schema named <code>tenant_schema</code>.
  In the <code>run_migrations_online</code> function in <code>env.py</code> i set up a schema_translate_map for <code>tenant_schema</code>.</p>
  
  <p>I expected <code>sqlalchemy</code> to translate this migration operation to run on the desired schema, however it seems like it tries to run sql queries using the schema <code>tenant_schema</code>.</p>
  
  <p>any ideas how to fix it ?</p>
  
  <p><strong>example</strong>:</p>
  
  <p>the upgrade function in the migration file:</p>
  
  <p><code>2018-09-05_17-28_247f3546088f_add_foo_column.py</code></p>
  
  <pre><code>def upgrade():
      op.add_column('derived_table', sa.Column('foo', sa.BigInteger(), nullable=True), 
                    schema='tenant_schema')
  </code></pre>
  
  <p>the <code>run_migrations_online</code> function:</p>
  
  <p><code>env.py</code></p>
  
  <pre><code>schema = 'other_name'  # normally i get the name as an argument from alembic
  def run_migrations_online():
      connectable = create_engine(get_url(), echo=True)
  
      with connectable.connect() as connection:
          # setting up the translation map
          conn = connection.execution_options(schema_translate_map={'tenant_schema': schema})
          context.configure(
              connection=conn,
              target_metadata=target_metadata,
              include_schemas=True,
              version_table_schema=schema,
              include_object=include_object,
          )
          with context.begin_transaction():
              context.run_migrations()
  </code></pre>
  
  <p>the exception (full traceback is too long and not that informative): </p>
  
  <pre><code>sqlalchemy.exc.ProgrammingError: (psycopg2.ProgrammingError) schema "tenant_schema" does not exist
   [SQL: 'ALTER TABLE tenant_schema.derived_table ADD COLUMN foo BIGINT'] 
  (Background on this error at: http://sqlalche.me/e/f405)
  </code></pre>
  
  <p>as you can see it tries to do <code>ALTER TABLE tenant_schema.derived_table</code> instead of the desired <code>ALTER TABLE other_name.derived_table</code></p>