title: Exception binding variables with cx_Oracle in python
body: <p>Okay, so I'm connected to an oracle database in python 2.7 and cx_Oracle 5.1 compiled against the instant client 11.2. I've got a cursor to the database and running SQL is not an issue, except this:</p>
  
  <pre>
  
      cursor.execute('ALTER TRIGGER :schema_trigger_name DISABLE',
                       schema_trigger_name='test.test_trigger')
  
  </pre>
  
  <p>or</p>
  
  <pre>
  
      cursor.prepare('ALTER TRIGGER :schema_trigger_name DISABLE')
      cursor.execute(None,{'schema_trigger_name': 'test.test_trigger'})
  
  </pre>
  
  <p>both result in an error from oracle:</p>
  
  <pre>
  
      Traceback (most recent call last):
        File "connect.py", line 257, in 
          cursor.execute('ALTER TRIGGER :schema_trigger_name DISABLE',
                      schema_trigger_name='test.test_trigger')
      cx_Oracle.DatabaseError: ORA-01036: illegal variable name/number
  
  </pre>
  
  <p>While running:</p>
  
  <pre>
  
      cursor.execute('ALTER TRIGGER test.test_trigger DISABLE')
  
  </pre>
  
  <p>works perfectly. What's the issue with binding that variable?</p>