devserver: Check whether devserver can run before rootfs update. BUG=chromium:718532 TEST=Test locally: provision a DUT with old version of ChromeOS. Change-Id: I3a7833ab2f81246015f8e228fd6de16509b93fa3 Reviewed-on: https://chromium-review.googlesource.com/500048 Commit-Ready: Xixuan Wu <xixuan@chromium.org> Tested-by: Xixuan Wu <xixuan@chromium.org> Reviewed-by: Allen Li <ayatane@chromium.org>