PIM 1463 : import product associations

TODO : 
 - [ ] processor cleanup + complete UT
 - [ ] add behat scenario to import products with associations

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [no]
Unit test passes: [yes]
Behat scenarios passes: [yes|no]
Checkstyle issues: [no]
ChangeLog updated: [no]
Fixes the following jira: PIM-1463