TMDM-6868: "Error when save values for nested multi-occurence element"

Fixes incorrect behavior of "isDeletingContainedElement" flag (especially in case where contained-type field contains mix of contained and not contained types).
Updates unit test to prevent data model conflicts when ran along others.
(https://jira.talendforge.org/browse/TMDM-6868)
Merge branch 'maintenance/5.6'