calico_framework should detect if tasks erroneously get the slave IP

[spikecurtis]

Mesos can currently fail to assign unique IPs to each container, but still pass the calico_framework.py test.

We need some test that can launch multiple tasks on a single slave and ensure that they have unique IP addresses.

After speaking with @djosborne, we have the following rough outline of how to extend the framework:

Define a method for each test that can examine all the tasks in the test and returns whether the test has failed or not. The scheduler should call this method every time we get a task update for the test. This allows us to write a test that ensures containers on the same slave have unique IP addresses.

The "default" implementation of the method just looks at tasks statuses and fails the test if any task has failed. This allows us to move that logic out of the scheduler, and thus we can also build tests that check specific conditions that should result in tasks failing.