Allow resizing kubernetes service worker-pools to 0

[TwoDCube]

IKS introduced the ability to resize an existing worker-pool to 0, so we have to allow that in the terraform provider as well.

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Output from acceptance testing:

% export TESTARGS='-run "(TestAccIBMContainerWorkerPoolBasic|TestAccIBMContainerWorkerPoolZeroSize|TestAccIBMContainerWorkerPoolInvalidSizePerZone)"'
% make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run "(TestAccIBMContainerWorkerPoolBasic|TestAccIBMContainerWorkerPoolZeroSize|TestAccIBMContainerWorkerPoolInvalidSizePerZone)" -timeout 700m
?   	github.com/IBM-Cloud/terraform-provider-ibm	[no test files]
?   	github.com/IBM-Cloud/terraform-provider-ibm/ibm/acctest	[no test files]
?   	github.com/IBM-Cloud/terraform-provider-ibm/ibm/provider	[no test files]

[...]

=== RUN   TestAccIBMContainerWorkerPoolBasic
--- PASS: TestAccIBMContainerWorkerPoolBasic (1244.19s)
=== RUN   TestAccIBMContainerWorkerPoolZeroSize
--- PASS: TestAccIBMContainerWorkerPoolZeroSize (1483.26s)
=== RUN   TestAccIBMContainerWorkerPoolInvalidSizePerZone
--- PASS: TestAccIBMContainerWorkerPoolInvalidSizePerZone (0.32s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/kubernetes	2732.484s

[...]