Refactor network topology JSONView class

This commit splits JSONView.get() method into submethods per resources.
It helps us when we add a config option to disable Router panel.

Unit tests for network topology JSONView is added to check the
refactoring doesn't introduce a regression.

Part of blueprint hide-router-panel-by-config
Change-Id: I859b8dd5fcdf86ddd25e91f8eba28ba6f4406a45
Merge "Refactor network topology JSONView class"