Replace the logic of ${fn:new-uuid()} in cnd with the newly hippo:identifiable mixin

In CMS-13587 in 2.18.5-h2-SNAPSHOT there has been added support for the hardcoded mixin
'hippo:identifiable' which will auto-create a UUID for property 'hippo:identifier'

In this issue we will

1. Add repository integration test for confirming correct behavior
2. Adjust all the existing logic making use of hst:qualifier (string) = '${fn:new-uuid()}' autocreated to replace it with the new hippo:identifier logic