Refactor the statistics of network in docker stats
Refactor the statistics of network in docker stats

For now docker stats will sum the rxbytes, txbytes, etc. of all
the interfaces.

It is OK for the output of CLI `docker stats` but not good for
the API response, especially when the container is in sereval
subnets.

It's better to leave these origianl data to user.

Signed-off-by: Hu Keping <ce660952c949ad2751377b45317337be332e80ac@huawei.com>

jwhonce/engine-api,docker/engine-api,mikedougherty/engine-api,thaJeztah/engine-api,rhatdan/engine-api,vdemeester/engine-api,msabansal/engine-api