network: Use neutron API for floating IP pollster.

The nova floating IP API doesn't include the tenant ID etc, so is not
very useful for a metering pollster. Fortunately, the corresponding
neutron API does include everything we need.

V2: Add missing oslo config import
V3: Fix various breakage
V4: Adjust author info

Signed-off-by: Chris Forbes <chrisf@catalyst.net.nz>
Add volume.size pollster to central agent