Add swarming utils.

The current test for this is naive, I need to manually move
chromeos-server51 out of prod, then force it not to kick off suite
for any request. Later I will add a test config for swarming,
and have a dedicated testing swarming proxy server for this.

BUG=chromium:728397
TEST=Ran a real test on chromeos-server51 (our suite-scheduler proxy
server) to provce a swarming task can be triggered:
https://chromeos-proxy.appspot.com/tasklist?c=name&c=state&c=created_ts&c=user&c=bot&et=1505502420000&f=pool%3Asuite-scheduler&l=50&n=true&s=created_ts%3Adesc&st=1505416020000

Change-Id: I4e99d94b854961be567f5034443723252553c0ca
3 files changed