Add system-info instance callback

Allow runners to update their own system information. Runners can now send
back os_name, os_version and agent_id back as part of a POST to
CALLBACK_URL/system-info/.

The goal is to get better info in regard to the actual OS that's running
and to move the agent_id from the status updates to the system-info callback.

The status updates should be used only to send back info about the status of
the installation process.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

params/requests.go

@@ -239,5 +239,5 @@ type UpdateEntityParams struct {
 type InstanceUpdateMessage struct {
 	Status  RunnerStatus `json:"status"`
 	Message string       `json:"message"`
-	AgentID *int64       `json:"agent_id"`
+	AgentID *int64       `json:"agent_id,omitempty"`
 }