autotest wifi: expose attenuation value in iperf measurement

 - record total attenuation as specified in test params ('total_db')
 - use 'unknown' for default value

BUG=chrome-os-partner:16471
TEST=Manual (ran test network_WiFiRateRange.001* to verify total attenuation
value was written to keyval file; ran test network_WiFiMatFunc.050* to verify
default value 'unknown' for total_attenuation)

Change-Id: I504a8d023eac6b06d6736f5a6cc69440f3baa754
Reviewed-on: https://gerrit.chromium.org/gerrit/38994
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: Tan Gao <tgao@chromium.org>
Tested-by: Tan Gao <tgao@chromium.org>
2 files changed