Commit e2a37be73da7bbc0ab562eabef69bfe69dc7cfa1 changed the behavior
of the OVS DVR agent so it will fail if can't get DVR config on start-up.
This patch adds a reno note describing this new behavior.
Related-Bug: #1536110
Change-Id: Idcdcdad9eba78ef5fbdae420e114a7ed793f3e43