Kill 'Skipping unknown group key: firewall_driver' log trace

This is done by trying to import the option first. If this
does not work, emit a warning instead as in most cases this is
harmless for a number of reasons: a) the service might not
even need the opt; b) if things do break down the line, we'll
see bigger traces; c) it's not gonna be long for this legacy
quantum/neutron stuff to be removed altogether.

Closes-bug: 1210121

Change-Id: I34917da9cb6117ee1d42140621c742f503279b6b
Merge "Kill 'Skipping unknown group key: firewall_driver' log trace"