Implement ethtool statistics PMDA

This PMDA collects ethtool statistics (ethtool -S) for
every NIC on the system that supports statistics collection.

The metrics are generated on the fly. Most drivers will output the exact same
statistics no matter how many NICs of the same type will be used. There are
exceptions though: Two NICs using the same driver could have different queue
numbers and hence a different number of metrics. The metrics are driver specific,
hence the PMNS will look like the following:
ethtool.tg3.rx_octets
ethtool.tg3.tx_octets
ethtool.tg3.rxq0_rx_bytes
...

Each metric will have zero or more instances depending on the number of NICs
on the system. For example:
ethtool.bnx2x.[0]: tx_ucast_packets
    inst [0 or "eth5"] value 0
    inst [1 or "eth4"] value 0

This PMDA needs statistics support in python-ethtool which is available
here: https://github.com/mbaldessari/python-ethtool