improve nova instance metering

Now we have a great load on Nova API because ceilometer-polling with "compute"
namespace will polling on Nova API periodically. This proposal aims to reduce
the load by abandoning the current way of metrics collection with periodically
polling.

The main idea of this proposal is to separate the resource metadata polling and
metrics polling to central namespace and compute namespace. compute namespace
ceilometer-polling will only focus on metering metrics about disk, cpu, net,
mem, etc. the resource metadata collected by central polling will can be
associated with samples by unique "resource_id", the resource_id can be
directly get from virt driver. these changes will avoid frequent Nova API
requests and make the instance related samples more lightweight.