Support data source of graylog_index_set

[suzuki-shunsuke]

This is a sub issue of #109 .

• https://www.terraform.io/docs/extend/writing-custom-providers.html#data-sources
• https://www.terraform.io/docs/configuration/data-sources.html

[suzuki-shunsuke]

@cameronattard

I have created the data source of IndexSet #111 .

Could you check whether this is what you want?

https://github.com/suzuki-shunsuke/go-graylog/blob/feat/issue-110/terraform/docs/data_source_index_set.md

I will also support other data sources such as stream.

Thanks!

[cameronattard]

@suzuki-shunsuke that's exactly what I want, thanks so much!