Fix metering iptables driver doesn't read root_helper param

With this patch the iptables driver instantiates the
iptables_manager with the correct root_helper value.

Change-Id: I9a37bae92dc12a8a78ea2d1b9fc5e995321ca322
Closes-bug: #1256036
Merge "Fix metering iptables driver doesn't read root_helper param"