Various fixes to circleci integration tests

[tomwilkie]

• Build the weave build container if we don't have it in the circle cache. That way, when it changes, we just flush the circle cache. It can be removed from the docker hub now.
• Add some timeouts to the test runs and GCE destroy vm function; this should help prevent stranded VMs.
• Checkin assert.sh instead of fetching it each time. This allows us to modify it to expose test run time. Its LGPL, so it should be fine.
• Add a simple scheduler to decide what tests run on which shard in circle. This should balance tests more accurately, and bring down total test run time down by about a minute or two.

[paulbellamy]

Maybe a moving average, instead of all of history?

[tomwilkie]

Update to use an exponential moving average.

[paulbellamy]

Do we need total_runs anymore?

[tomwilkie]

I do not, but I thought it would be nice to keep.

[paulbellamy]

LGTM, then.

[tomwilkie]

Tests on the different shards took 03:13 and 03:02 - yay for scheduler!