Expose shouldProvision to provisioners via new controller interface

[aka-bo]

Added new controller.Qualifier interface which is implemented by
any controller that has a ShouldProvision method. When implemented,
the provisioner's ShouldProvision method is called by lib controller's
shouldProvision with a return value of false from the former
resulting in the same from the latter.

This allows provisioners the opportunity to ignore individual claims
prior to attaining leadership for the claim, avoiding uneccessary
election cycles when multiple provisioners are serving the same storage
class.

[rootfs]

/assign @jsafrane

[jsafrane]

This allows provisioners the opportunity to ignore individual claims prior to attaining leadership for the claim

Do you have a use case where this is useful? So far all instances of one provisioner were equal and each of them could provision all PVCs.

[aka-bo]

So far the most applicable use cases we've encountered are centered around capacity. While all instances of a provisioner start out on equal footing, over time each instance has the potential to exhaust the capacity of its backing storage medium at different rates, e.g., available disk space or--in the case of NFS--max exports. When this happens today an instance will still attempt to gain leadership for a PVC even when it lacks the ability to provision, leading to significant delays in provisioning.

[jsafrane]

Ok, that sound useful.
/lgtm

@childsb, please merge.