Feature/214 adding unit tests node model manager

This MR purpose is to add more unit tests to model manager. Unit tests have been redesigned and some issues have been fixed.

Main issues fixed:

1. rename tests name
2. do not call environ['VARIABLE']=some_variable but rather self.values['VARIABLE']=some_variable where self.variable = copy.deepcopy(environ)
3. other tests have been added
4. created another Exception FedbiomedModelManagerException

other issues that may require reviewer attention:

• should test be skipped if some PermissionError or MemoryError are triggered when running tests?
• variable name?
• patches