Add "tearDownClass" call from parent class
Add "tearDownClass" call from parent class

The "NeutronBaseTest" class should call
"tearDownClass" method from base class
to initialize "error_msg". The reason is
"AttributeError" for derived classes
if "error_msg" was not initialized.

Change-Id: Ifb78e7902b37f70f6adea6a214f8e440b0ec767f
Closes-Bug: 1604315

stackforge/fuel-ostf,stackforge/fuel-ostf