provider/kubernetes: Add support for persistent_volume

[radeksimko]

Sorry for the 2,4k LOC 🙈 , but vast majority of it is the schema & related expanders/flatteners.

Test plan

I avoided writing tests for every single volume source provider for two reasons - avoid ⬆️ LOC and because K8S API doesn't seem to care about anything except optional/required fields. That is also why most of the configs have non-sense volume IDs etc. which don't exist. I suspect K8S would only failed to schedule a pod with such volume - i.e. it is just postponing the error/validation to latest moment.

I did try to create PV for each volume source provider manually though and that worked 👌 .

make testacc TEST=./builtin/providers/kubernetes TESTARGS='-run=TestAccKubernetesPersistentVolume_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/02 18:51:04 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/kubernetes -v -run=TestAccKubernetesPersistentVolume_ -timeout 120m
=== RUN   TestAccKubernetesPersistentVolume_basic
--- PASS: TestAccKubernetesPersistentVolume_basic (5.37s)
=== RUN   TestAccKubernetesPersistentVolume_importBasic
--- PASS: TestAccKubernetesPersistentVolume_importBasic (1.68s)
=== RUN   TestAccKubernetesPersistentVolume_volumeSource
--- PASS: TestAccKubernetesPersistentVolume_volumeSource (2.63s)
=== RUN   TestAccKubernetesPersistentVolume_cephFsSecretRef
--- PASS: TestAccKubernetesPersistentVolume_cephFsSecretRef (1.91s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/kubernetes	11.662s

[grubernaut]

LGTM, pending fixed merge conflicts

[ghost]

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