Allow async call to stage artifacts.

Add async argument to allow async call to stage artifacts. Default to False. In
async mode, client will poll devserver.is_staged method to check if artifacts
are staged.

BUG=chromium:249426
TEST=local devserver with link like:
http://localhost:8093/stage?archive_url=gs://chromeos-image-archive/lumpy-release/R30-4408.0.0/&artifacts=autotest,test_suites&async=True
http://localhost:8093/is_staged?archive_url=gs://chromeos-image-archive/lumpy-release/R28-4100.7.0&artifacts=autotest,test_suites
run_suite from local setup to stage a build that's not in the devserver.

Change-Id: I7777d5d25e8870fef7edf8a9084bca18f3624c46
Reviewed-on: https://gerrit.chromium.org/gerrit/64310
Reviewed-by: Chris Sosa <[email protected]>
Commit-Queue: Dan Shi <[email protected]>
Reviewed-by: Dan Shi <[email protected]>
Tested-by: Dan Shi <[email protected]>
3 files changed