provider/alicloud: Fix create PrePaid instance (#13661)

[pragkent]

Fix creating PrePaid instance error

Use different IoOptimized value for PrePaid instance and PostPaid instance

Fixes: #13661

[pragkent]

make testacc TEST=./builtin/providers/alicloud TESTARGS=-run='TestAccAlicloudInstance_basic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/14 19:42:20 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/alicloud -v -run=TestAccAlicloudInstance_basic -timeout 120m
=== RUN TestAccAlicloudInstance_basic
--- PASS: TestAccAlicloudInstance_basic (102.24s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/alicloud 102.259s

[pragkent]

I have successfully created a few PrePaid instances with modified code.

Since the PrePaid instance is far more expensive to create and it will exist for a month, I think it might not be a good idea to create a automated test case for this. So I just run the basic test case to make sure the PostPaid instances do not complain about it.

If you need anything else, please let me know.

[stack72]

LGTM! Thanks for this :)

[ghost]

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