Fix measurement docs to correctly represent Existance meters

There are various meters in the measurement docs that
wrongly note the meter as Duration instead of its
existance. The meters fixed include network, port,
router, subnet, floating ip, volume.

fixes bug #1252988

Change-Id: I45a84769d6ee46d53e580ae08cbbf32ed20562b1
Merge "Fix measurement docs to correctly represent Existance meters"