CHERRY-PICK: factory_install: Skip download if source URL is empty

When complete.gz is not served on the server, its source URL is not set.
In this case, installer will connect to default URL http://<host
ip>:<port>/update, but this URL is not used in Umpire server.
This CL fixes this issue such that if the source URL of a channel is not
set, it should be skipped.

BUG=chrome-os-partner:25791
TEST=download image from Umpire without complete scripte.

Original Change-Id: Iaa2fc535b7403b651816f78a0709c70dfdaa0c61
Original Reviewed-on: https://chromium-review.googlesource.com/202904
Original Reviewed-by: Rong Chang <rongchang@chromium.org>
Original Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Original Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Original Commit-Queue: Cheng-Yi Chiang <cychiang@chromium.org>
(cherry picked from commit 2126e680c22b2d4ffb6cc72fffacde97bf88ec64)

Change-Id: Ib214e4bd43288c76785efbc0b666858bfa8fb770
Reviewed-on: https://chromium-review.googlesource.com/203917
Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
1 file changed