Downloader: Turn on backgrounding of artifacts not needed early on.

This change turns on the background=True functionality of the
downloader.  This allows the autotest server to install images
on to DUT's while the downloader is staging larger artifacts like
autotest andn N+1 delta payloads.

BUG=chromium-os:27285
TEST=Ran with same cli command tested autotest change with.
CQ-DEPEND=Iae2a20762c565b8e6d440c9a501c1f99c73129ed

Change-Id: I0947f60c0a85319ee3e7fd6019c3e2aef5d8725d
Reviewed-on: https://gerrit.chromium.org/gerrit/19345
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Tested-by: Scott Zawalski <scottz@chromium.org>
Commit-Ready: Scott Zawalski <scottz@chromium.org>
diff --git a/devserver.py b/devserver.py
index 2529581..3b606ea 100755
--- a/devserver.py
+++ b/devserver.py
@@ -227,7 +227,7 @@
     if not archive_url:
       raise DevServerError("Didn't specify the archive_url in request")
 
-    return_obj = downloader_instance.Download(archive_url)
+    return_obj = downloader_instance.Download(archive_url, background=True)
     self._downloader_dict[archive_url] = downloader_instance
     return return_obj
 
@@ -378,7 +378,7 @@
                     dest='validate_factory_config',
                     help='Validate factory config file, then exit.')
   parser.add_option('-l', '--log-dir', default=None,
-                    help=('Specify a directory for error and access logs. ',
+                    help=('Specify a directory for error and access logs. '
                           'Default None, i.e. no logging.'))
   (options, _) = parser.parse_args()