provider/kubernetes: Use PATCH to update namespace

[radeksimko]

Reasons are explained in #12945

Test plan

make testacc TEST=./builtin/providers/kubernetes TESTARGS='-run=TestAccKubernetesNamespace_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/28 07:09:39 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/kubernetes -v -run=TestAccKubernetesNamespace_ -timeout 120m
=== RUN   TestAccKubernetesNamespace_basic
--- PASS: TestAccKubernetesNamespace_basic (17.25s)
=== RUN   TestAccKubernetesNamespace_importBasic
--- PASS: TestAccKubernetesNamespace_importBasic (6.19s)
=== RUN   TestAccKubernetesNamespace_generatedName
--- PASS: TestAccKubernetesNamespace_generatedName (5.87s)
=== RUN   TestAccKubernetesNamespace_importGeneratedName
--- PASS: TestAccKubernetesNamespace_importGeneratedName (6.75s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/kubernetes	36.132s

[grubernaut]

LGTM

[ghost]

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