Chef provider: add content_file for all resources

[hashibot]

This issue was originally opened by @partamonov as hashicorp/terraform#4581. It was migrated here as part of the provider split. The original body of the issue is below.

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With current implementation all roles/data bags items/envs should be specified inside .tf file.

This mean that to use it you have to move all roles/data bags items/envs inside .tf file, but in many cases these roles/data bags items/envs are stored in repositories and better approach (or additional available) is to read from file, with file() interpolation function.

• chef_data_bag_item -> content_file "file('path/to/data_bag_item.json')"
• chef_environment -> content_file, should discard all other attributes
• chef_role -> content_file, should discard all other attributes

Also for chef_data_bag_item we can use template_file to get content, but it's better to add this to documentation example.