Add support for inter-pod affinity strategy

[concaf]

For pods with podAffinity set using preferredDuringSchedulingIgnoredDuringExecution, it might be possible that at the time of scheduling on the current node, no pod with the set labels were running, but still the pod got scheduled on the current node since the nature of the affinity was preferred and not required.

In such a case, if the descheduler is run, it can do the following -

1. finds pods running with podAffinity set using preferredDuringSchedulingIgnoredDuringExecution
2. checks if the pods found in 1 are scheduled on the desired node or not
3. if not, descheduler checks on other schedulable nodes if the desired pods are running where this podAffinity condition can be met
4. if such a node is found, descheduler evicts the pod (and hopefully the scheduler schedules it on the desired node 🎉)

Maybe we could have a policy file describing the strategy like -

apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "RemovePodsViolatingPodAffinity":
     enabled: true

Does it make sense to support such a strategy?

[aveshagarwal]

@containscafeine as I explained here: #46 (comment), first I'd suggest to address requiredDuringSchedulingIgnoredDuringExecution.

[ravisantoshgudimetla]

@aveshagarwal - Can you assign appropriate milestone for this? We have pod anti-affinity strategy, do you want to include this in 0.4 release?

[komljen]

I would like to have the ability to set podAffinityType for RemovePodsViolatingInterPodAntiAffinity strategy, similar to nodeAffinityType where we can set required or preffered affinity types. Does this issue covers that scenario?

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