erp5_catalog: Use getattr to get catalog attributes

Both SQL Catalog and the new ERP5 Catalog have all properties as their
attributes also, so its better to user `getattr` as it'll maintain
consistency betwen both.

This was required as in ERP5 Catalog, these properties are multivalued
propeties, so they have multiple accessors, so if we use 'getProperty',
we won't get the objects we desired. So, instead of relying on that, why
not just use the attributes.