ProvisionController support for SharedInformers

[zcahana]

This PR adds support for running the ProvisionController with SharedInformers.

This is achieved by introducing a new constructor, NewProvisionControllerWithConfig() that enables to optionally pass in shared informers; The existing constructor, NewProvisionController(), as well as its behavior, are left as they were.

Resolves #546.

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[zcahana]

@k8s-ci-robot I've signed the CLA.

[zcahana]

/cc @wongma7

[zcahana]

I've refactored the code to be consistent with the functional options pattern:

• Introduced 3 new option functions: ClaimsInformer(), VolumesInformer(), and ClassesInformer(),
  to be used with NewProvisionController().
• Removed the NewProvisionControllerWithConfig() constructor mentioned in the beginning of this PR.

[zcahana]

@wongma7 did you get a chance to look at this?

[rootfs]

/assign @jsafrane
wait till #578 is resolved

[zcahana]

I've removed the commit which vendored the additional client-go/informers/* packages (as these were added as part of #578), and rebased.

[jsafrane]

/lgtm

[jsafrane]

/approve no-issue
Checking if there is merge bot...