add ifspeed metric to track interface speed

This change adds the linux.net.ifspeed and os.net.ifspeed metrics to track ethernet interface speed.  This can be used in other calculations (for example, capacity measurements)
Instead of creating a new tagset here, I'd make one below line 77:
`tags := opentsdb.TagSet{"iface": intf}`
and then reference that when you need it. This will make the newlines needless.