shill: add constant for DHCP lease expiry time

BUG=b:26046393
TEST=unit test, CDRouter tests suite

Change-Id: I5d229b9774bed330eac2c18b481a21979b7e9c1f
Reviewed-on: https://chromium-review.googlesource.com/316411
Commit-Ready: Timothy Jennison <tjennison@chromium.org>
Tested-by: Timothy Jennison <tjennison@chromium.org>
Reviewed-by: Zeping Qiu <zqiu@chromium.org>
1 file changed