cellular: Migrate network_3GDisableWhileConnecting to test_environment.

Also migrated to shill proxy and cleaned up pylint errors.

BUG=chromium:415293
TEST=network_3GDisableWhileConnecting

Change-Id: I0d1cd583b3217fccc8102d1b6044fe56f295dc1e
Reviewed-on: https://chromium-review.googlesource.com/219848
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
14 files changed