Automatic merge from submit-queue

Allow disabling of dynamic provisioning

Allow administrators to opt-out of dynamic provisioning.  Provisioning is still on by default, which is the current behavior.

Per a conversation with @jsafrane, a boolean toggle was added and plumbed through into the controller.  Deliberate disabling will simply return nil from `provisionClaim` whereas a misconfigured provisioner will continue on and generate error events for the PVC.

@kubernetes/rh-storage @saad-ali @thockin  @abhgupta