[BUGFIX] Additional identity field are missing

I have a form with children object, when the form is rendered I have 3
hidden fields with the different __identity value, but after a
validation error I only see on hidden field with the identity of the
main object.

I my case, this cause a big trouble, has Flow build a new object, and
doctrine is configured to cascade persist on a one to one relation. This
cause the child object data to be "replaced" by the new object data, and
I loose all the property of the object that are not in the form.

Change-Id: Ib8d339fe76e9e94396e1064121282f8037f77b4e
Resolves: #45184
Releases: 2.0, master