autotest: Expose subnet prefix in site_linux_router

This lets tests check that the IP address they have been assigned
looks like it belongs in the correct IP range.

BUG=chromium:294856
TEST=check11b still passes

Change-Id: Ifbb22a7ecb2c89c6641694238c42d0f9fd334a86
Reviewed-on: https://chromium-review.googlesource.com/170004
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
1 file changed