mv_machine_learning: use DEFAULT_MODEL_NAME for face detection task

mv_machine_learning: use DEFAULT_MODEL_NAME for face detection task

[Issue type] : new feature

Use the meta file for face detection task to set the default model

With this patch, a proper concrete class object will be created
at the constructor of FaceDetectionAdapter class according to the default
model name which is parsed from face_detection.json file.

And also this patch calls the member function, create, to create the concrete
class object corresponding to the default model name instead of creating
a default concrete class object directly.

Finally, we can create user-desired concrete class object without rebuilding
by simply modifying the meta file, face_detection.json.

Change-Id: Iaddd0a820180332707852bbea51afcafb49238fa
Signed-off-by: Inki Dae <inki.dae@samsung.com>