[autotest] Allow test to pick wireless ssid based on host's subnet

This change allows a test to choose wireless ssid from shadow config based on
its subnet. Global config has a fixed CLIENT/wireless_ssid config for test
to get the wireless ssid. To support different host with different wireless
ssid, we need to allow more flexible config, e.g.,
[CLIENT]
wireless_ssid: ssid_0
wireless_ssid_1.2.3.4/24: ssid_1
wireless_ssid_4.3.2.1/16: ssid_2

For hosts in subnet 1.2.3.4/24, ssid_1 will be used.
For hosts in subnet 4.3.2.1/16, ssid_2 will be used.
For hosts in other subnets, ssid_0 will be used.

BUG=chromium:577506
TEST=unittest

Change-Id: I03ee67c1d9efc40c78ab4947895b2dac886854dd
Reviewed-on: https://chromium-review.googlesource.com/324560
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Dan Shi <dshi@google.com>
5 files changed