dropAllForeignKeyConstraints does not work on Oracle

Using this tag on Oracle where the constraint has multiple columns results in ORA-02443: Cannot drop constraint  - nonexistent constraint.  

The problem seems to be caused by the generated SQL used to retrieve the list of foreign keys to drop.  Since the columns are also returned in the query, you get one row constraint and column returned.  This is causing liquibase to attempt to drop the constraint multiple times.  

Created jira issue CORE-921