Avoid discarding precision of metering data

Fixes bug 1241467

Narrow-casting statistical aggregates to int before inserting
into the resource usage line charts had the effect of unnaturally
smoothening the trend. This was especially apparent for meters
defined over a narrow range (such as cpu_util ranging from 0.0%
to 100.0%).

We now maintain floating point precision for these data.

Change-Id: If6141a4f66aa0078d38aa9f53d913aca5684ad94
(cherry picked from commit 4b973c2fb49b3a1a0d38c3788f45b3af076625d0)
Merge branch 'stable/havana' of https://github.com/openstack/horizon into stable/havana_routerrules