provider: Source SCALEWAY_ORGANIZATION variable correctly

[radeksimko]

Similar to https://github.com/terraform-providers/terraform-provider-scaleway/pull/113 this fixes another bug introduced in https://github.com/terraform-providers/terraform-provider-scaleway/pull/111

Without this patch (and #113 ), the provider unfortunately can't authenticate against the API via ENV variables, which can also be seen by running any acceptance test:

=== RUN   TestAccScalewayBucket
--- FAIL: TestAccScalewayBucket (5.27s)
    testing.go:538: Step 0 error: Error applying: 1 error(s) occurred:
        
        * scaleway_bucket.base: 1 error(s) occurred:
        
        * scaleway_bucket.base: StatusCode: 400, Type: invalid_request_error, APIMessage:  [31mValidation Error [0m, Details: map[organization:[SCALEWAY_ORGANIZATION is not a valid UUID.]]
FAIL

[nicolai86]

I immediatly switched to using the .scwrc file - should have been more careful.

Thanks so much!