Clarify heartbeat code

heartbeats are not sent for self-hosted cloudrons (only managed ones)

src/cloudron.js

@@ -12,7 +12,7 @@ exports = module.exports = {
     dnsSetup: dnsSetup,
     getLogs: getLogs,
 
-    sendHeartbeat: sendHeartbeat,
+    sendCaasHeartbeat: sendCaasHeartbeat,
 
     updateToLatest: updateToLatest,
     reboot: reboot,
@@ -440,8 +440,8 @@ function getConfig(callback) {
     });
 }
 
-function sendHeartbeat() {
-    if (config.provider() !== 'caas') return;
+function sendCaasHeartbeat() {
+    assert(config.provider() === 'caas', 'Heartbeat is only sent for managed cloudrons');
 
     var url = config.apiServerOrigin() + '/api/v1/boxes/' + config.fqdn() + '/heartbeat';
     superagent.post(url).query({ token: config.token(), version: config.version() }).timeout(30 * 1000).end(function (error, result) {