Bug fix truncate method on Mysql 5.5.30

## Context
Upgrade Mysql from 5.0.92 to 5.5.30.

## Description
In Mysql 5.5.30 foreign keys are checked before a truncate table and it fails if there is a foreign key on the table EVEN if there's no data.

Now, we disable foreign keys check before truncating a table by setting the ```foreign_key_checks``` session variable.