Refactor wait_for_{pods, pvcs, pvc} and {pods, pvcs, pvs} files

[wojtek-t]

We're accumulating a lot of code duplication for different types:

• pods:
  https://github.com/kubernetes/perf-tests/blob/master/clusterloader2/pkg/measurement/util/pods.go
  https://github.com/kubernetes/perf-tests/blob/master/clusterloader2/pkg/measurement/common/wait_for_pods.go
• pvcs
  https://github.com/kubernetes/perf-tests/blob/master/clusterloader2/pkg/measurement/util/pvcs.go
  https://github.com/kubernetes/perf-tests/blob/master/clusterloader2/pkg/measurement/common/wait_for_pvcs.go
• pvs
  https://github.com/kubernetes/perf-tests/blob/master/clusterloader2/pkg/measurement/util/pvs.go
  https://github.com/kubernetes/perf-tests/blob/master/clusterloader2/pkg/measurement/common/wait_for_pvs.go

Those should be refactor to share as much code as possible

[wojtek-t]

How I would see that:

• the files from /util directory should implement some interfaces (Diff, ComputeStatus, etc.)
• the wait_for-* should simply take the interface and use that (this one should hopefully be possible to implement just once and not even have any customizations for)

[wojtek-t]

@krzysied - FYI

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[wojtek-t]

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