title: alembic create_table, check if table exists
body: <p>I have an alembic upgrade script that creates a table, however I don't want it to create the table if it already exists.</p>
  
  <p>According to the <a href="http://alembic.readthedocs.org/en/latest/ops.html#alembic.operations.Operations.create_table" rel="noreferrer">alembic doc</a>, I can pass in keyword args to <code>op.create_tables</code> that are acceptable to <a href="http://docs.sqlalchemy.org/en/rel_1_0/core/metadata.html#sqlalchemy.schema.Table" rel="noreferrer">sqlalchemy.schema.table</a>, so I'm using the <code>keep_existing</code> keyword:</p>
  
  <pre><code>op.create_table('foo_model',
    sa.Column('foo_id', sa.Integer(), nullable=False),
    sa.Column('foo_str', sa.String(length=255), nullable=True),
    sa.PrimaryKeyConstraint('foo_id'),
    keep_existing= True
    )
  </code></pre>
  
  <p>However I'm still getting the table already exists error.</p>
  
  <pre><code>sqlalchemy.exc.InternalError: (InternalError) (1050, u"Table 'foo_model' already exists") '\nCREATE TABLE foo_model (\n\tfoo_id INTEGER NOT NULL AUTO_INCREMENT, \n\tfoo_str VARCHAR(255), \n\tPRIMARY KEY (foo_id)\n)\n\n' ()
  </code></pre>