Fixes several issues with integration tests.

* Ports and URLs used in tests are now passed from the test harness into the tests.
* The sheduler is now started with an explicit port assignment.
* The temporary rootfs from the previous run is now cleaned if it is present.
* The harness now waits until the scheduler starts responding before it launches the scheduler tests.
Merge branch 'master' of github.com:soundcloud/harpoon into recover-ports