Network model add enable wavelength unit test

Network model add enable wavelength unit test

- Enable and fix NetworkModelWavelengthServiceTest.
- Remove no more used PathDescriptionUtils test class
- Add a stub yang model for path-description in test-common to define
path description as a container and use it to parse
path_description.json test file. Using a json file avoid to have big
test classes to instantiate yang object.

JIRA: TRNSPRTPCE-230
Signed-off-by: manuedelf <emmanuelle.delfour@orange.com>
Change-Id: If843edb8116cb7ac60cffa3cd25544e9a251c88a