Modify dynamic suite pathway to allow some artifacts to be staged in the bgnd.

This change modifies the site_rpc_interface->dynamic_suite.py workflow to allow
autotest to stage larger build components while imaging devices. It uses
the download/wait_for_status interface provided by the devserver to
accomplish it.

BUG=chromium-os:27285
TEST=All effected unittests pass. Staging larger test on local devserver in
parallel.

Change-Id: Iae2a20762c565b8e6d440c9a501c1f99c73129ed
Reviewed-on: https://gerrit.chromium.org/gerrit/19328
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>
5 files changed