Update measurement docs for VPNaaS

Change-Id: Icbb49bb297e1e21d9a14bdfacc234da96866f793
Partially-Implements: blueprint ceilometer-meter-vpnaas
Update measurement docs for FWaaS

Change-Id: I8d600ffb46f3f6ed0a5a6087805b269632b5b340
Partially-Implements: blueprint ceilometer-meter-fwaas
Merge "Update measurement docs for VPNaaS"
Merge "Update measurement docs for FWaaS"