pdm: support default image in Service,Component Individual Variations

in c361a8c3 (erp5_pdm: improve product invividual variation, 2020-03-09)
we changed to allow default images in product individual variation and modified
IndividualVariation_view to add a file upload field to upload an image, but this
form is shared with other individual variations portal types.

Without setting the default image property sheet and allowing Embedded Files as
sub-documents, saving the form was causing an error.
5 jobs for <span class="ref-name">fix/pdm-variation-save-error</span> in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
running ERP5.CodingStyleTest-Master

3166:55:22

passed ERP5.CodingStyleTest-TestRunner2

00:44:27

running ERP5.UnitTest-Master.Medusa

3166:55:23

failed ERP5.UnitTest-TestRunner2

01:17:08

running SlapOS.Eggs.UnitTest-Master.Python2

3166:55:23