Add ebsOptimized and productCodes as valid InstanceAttributes

Currently, `conn.get_instance_attribute` and `instance.get_attribute` return an empty dict if you request the 'ebsOptimized' or 'productCodes' attributes, despite the documentation stating quite clearly that they are valid. This one-line patch adds both attributes to the  `InstanceAttribute.ValidValues` class-level list.

Before this patch:

    In [3]: hp.connection.get_instance_attribute('i-4e2c713d', 'ebsOptimized')
    Out[3]: {}
    
    In [4]: hp.connection.get_instance_attribute('i-4e2c713d', 'productCodes')
    Out[4]: {}

After this patch:

    In [3]: hp.connection.get_instance_attribute('i-4e2c713d', 'ebsOptimized')
    Out[3]: {u'ebsOptimized': True}
    
    In [4]: hp.connection.get_instance_attribute('i-4e2c713d', 'productCodes')
    Out[4]: {u'productCodes': None}

Thank you,

Charles