Fix cells JSON example

The previous cells JSON example was wrong. This patch
updates it to be the same as that in etc/nova/cells.json
from the nova codebase.

Change-Id: I0d692edd11a5a3921fc7b2f9e463899984277df6
Closes-Bug: 1240800
(cherry picked from commit ae06e3b8bc93645fc0ca2f19a57be7a35a77c8a2)
Merge "Fix cells JSON example" into stable/havana