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
Fix docs on what an instance meter represents

instance meter is a gauge which meters an instance
itself. The current description of duration of
instance is misleading.

fixes bug #1266313

Change-Id: I902606928fc59f5663d638d05f237fe3e5e1175d
Merge "Fix docs on what an instance meter represents"
Merge "Fix measurement docs to correctly represent Existance meters"