http: add PrimaryDatacenter to the /v1/agent/self response

This field is available in DebugConfig, but that field is not stable and could change at any time.

The consul-k8s needs to be able to detect the primary DC for tests, so adding this field to the
stable part of the API response.

agent/agent_endpoint.go

@@ -75,6 +75,7 @@ func (s *HTTPHandlers) AgentSelf(resp http.ResponseWriter, req *http.Request) (i
 
 	config := struct {
 		Datacenter        string
+		PrimaryDatacenter string
 		NodeName          string
 		NodeID            string
 		Revision          string
@@ -82,6 +83,7 @@ func (s *HTTPHandlers) AgentSelf(resp http.ResponseWriter, req *http.Request) (i
 		Version           string
 	}{
 		Datacenter:        s.agent.config.Datacenter,
+		PrimaryDatacenter: s.agent.config.PrimaryDatacenter,
 		NodeName:          s.agent.config.NodeName,
 		NodeID:            string(s.agent.config.NodeID),
 		Revision:          s.agent.config.Revision,