Do not parse options.android_build_credential if the file does not exist.

So moblab upstart job can have the arg specified to a fixed file even if the
file is not existed yet.

BUG=chromium:570705
TEST=verify with a local moblab build

Change-Id: I339343bb61b3eb40885fbb2f339ecb94bf2f7fb1
Reviewed-on: https://chromium-review.googlesource.com/320416
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Simran Basi <sbasi@chromium.org>
1 file changed