[WIP] PIM-1463 : import associations of products

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [no]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [yes]
Fixes the following jira:
 - PIM-1535
 - PIM-1464
 - PIM-1523
 - PIM-844
 - PIM-848