Add generic_config to node template resource

[yamamoto-febc]

This is re-post of #75.

This PR was still reviewing, but that repo had transfered and archived, so I re-posted here.

[yamamoto-febc]

The following points were pointed out in the last review.

Where are MarshalJSON/UnmarshalJSON used?

• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)

I think there are used when communicate with Rancher APIs.

https://github.com/rancher/norman/blob/master/clientbase/ops.go

Defination of struct or fields

• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)

I think struct NodeTemplate and CloudCredential are used as parameter to Rancher APIs.
genericConfig and genericCredentialConfig are not sent directly to the Rancher API, they are converted to the appropriate form by MarshalJSON/UnmarshalJSON, so I think that it is unnecessary to make them exported and json/yaml tag.

NodeDriver's ID and Name

I think Rancher has two types of NodeDriver.

• 1: builtin: Built-in driver has the same value for both ID/Name(e.g.: amazonec2).
• 2: custom: For other drivers, ID is generated by Rancher, and Name is calculated from the docker-machine driver name.

And this PR supports both types of NodeDriver.
So I think that these points are unnecessary.

• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)
• Added rancher2_registry resource and datasource #75 (comment)