Stabilize the /v1/agent/self "Config" output

[slackpad]

The Config sub-key has always just been a dump of an internal structure, so it hasn't been stable. We need to look internally and externally (hashicorp/nomad#3324) on places that depend on fields here and make a small, stable set of fields under Config that we document:

• NodeName string
• Datacenter string
• Revision string
• Server bool
• Version string

These are sourced from:

• https://github.com/hashicorp/nomad/blob/master/client/fingerprint/consul.go#L68-L72
• https://github.com/hashicorp/consul/blob/master/api/agent.go#L215
• https://github.com/hashicorp/consul/blob/master/command/exec.go#L193-L201
• https://github.com/hashicorp/consul/blob/master/command/rtt.go#L84

This test will be broken and needs an update - https://github.com/hashicorp/consul/blob/master/api/agent_test.go#L640-L647.

We should move the existing Config contents to DebugConfig and document that this is for debugging use only and is subject to change.

Once this is complete, we need to open a PR against https://github.com/terraform-providers/terraform-provider-consul/blob/master/consul/data_source_consul_agent_self.go that removes unsupported fields and notes that this data source will be deprecated in a future release of the provider.

[magiconair]

• https://github.com/hashicorp/nomad/blob/master/client/fingerprint/consul.go#L68-L72: Datacenter, NodeName, Revision, Version
• https://github.com/hashicorp/consul/blob/master/api/agent.go#L215: NodeName
• https://github.com/hashicorp/consul/blob/master/command/exec.go#L193-L201: Datacenter, NodeName
• https://github.com/hashicorp/consul/blob/master/command/rtt.go#L84: Datacenter, NodeName