shill: OpenVPNDriver: Allow server to override routes

If the remote OpenVPN server pushes a redirect-gateway or
redirect-private argument to the client, we should ignore
the client request to drop the gateway route, since in this
case a split tunnel will likely not be possible.

BUG=chromium:370460
TEST=Unit tests

Change-Id: I1456f2e40d1dc500bbd7ed741c2e0589bd981a24
Reviewed-on: https://chromium-review.googlesource.com/199056
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>