Add api test for DVR coverting back to centralized

This patch adds a test case for DVR routers which converts a centralized
router to distributed firstly, followed by making a transition back to
centralized. It checks whether the attributes are updated or retained as
expected.

Related-Bug: #1780094
Change-Id: Ida6e75d0215283c15b7a1c0ce0f473169afe8a66
1 file changed