Add method to get iptables traffic counters

Add methods to iptables_manager to get traffic counters.
this is a part of the blueprint bandwidth-router-measurement

Change-Id: I6485d1239085e46398126c81e674e399952a4597
Iptables metering driver

this is a part of the blueprint bandwidth-router-measurement

Change-Id: I37e4dc5abeaca4e13b32155bb7e2f07883ef9b2d
Merge "Add method to get iptables traffic counters"
Merge "Iptables metering driver"