Metric 'memory.usage' depends on balloon driver,
and shows the memory usage inside guest-os,
while 'memory.resident' represents volume of RAM
used by the instance on the physical machine
So, it is more reasonable to use 'memory.resident'
when calculating node utilization
by gathering instances utilization of the node.
Change-Id: I12dff5176bcf6cb103aa44cafd54f9ecd7170864