Update dev docs to include notification-agent

I was reading through the developer docs and noticed they had not been
updated for the new ceilometer-agent-notification service. This patch
adds the right command to put into the localrc. The command comes
straight out of devstack/lib/ceilometer.

Change-Id: I23b917b1c48b6474de2b29901fa039d8353f0eb0
Merge "Update dev docs to include notification-agent"