Handle race conditions in exclusive placement

[danielvegamyhre]

Now that #310 has been addressed via the pod webhook approach, we need to handle certain race conditions which can occur in this strategy.

Consider the following race condition:

1. Leader of Job 1 getting killed right after we set the nodeSelector on its follower pods
2. At the same time, leader of Job 2 lands on the same slice before any of Job 1's followers get to schedule on it. In this case Job 1's followers will have a nodeSelector on the node pool, but will never schedule because of the anti-affinity of Job 2's leader.

[danielvegamyhre]

/assign