Defer evaluation of defaults for ObjectTypeField

[pi-sigma]

The defaults for the ObjectTypeField are retrieved with a request to the Objecttypes API. This creates a problem for certain use cases. In OIP, ObjectTypeField is set set on a Django CMS plugin, with the effect that the field is evaluated immediately when the class is imported (rather than when the form is constructed, where the default values for the field are actually needed). This makes testing difficult because it is impossible (?) to mock out the relevant request.

To be determined if this should be fixed in the library or in the client (e.g. OIP).