Allow to resize a droplet permanently (i.e. apply disk resize)
if previously it was resized temporarily (CPU and RAM only).
Fixes: #13007
```
$ make testacc TEST=./builtin/providers/digitalocean TESTARGS='-run=TestAccDigitalOceanDroplet_ResizeOnlyDisk'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/25 03:54:23 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/digitalocean -v -run=TestAccDigitalOceanDroplet_ResizeOnlyDisk -timeout 120m
=== RUN TestAccDigitalOceanDroplet_ResizeOnlyDisk
--- PASS: TestAccDigitalOceanDroplet_ResizeOnlyDisk (198.62s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/digitalocean 198.638s
```
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