fix verify registered agents

Use API, agents are rendered by SPA now

Author: tomzo

test/integration/default/serverspec/go_autoregister_spec.rb

@@ -7,7 +7,7 @@
   describe command('curl -L localhost:8153/go') do
       its(:stdout) { should contain('/go/home') }
   end
-  describe command('curl -L http://localhost:8153/go/agents') do
+  describe command('curl -H \'Accept: application/vnd.go.cd.v4+json\' localhost:8153/go/api/agents') do
     its(:stdout) { should contain('/var/lib/go-agent') }
     its(:stdout) { should contain('Idle') }
   end

test/integration/golangagent/serverspec/go_autoregister_spec.rb

@@ -7,7 +7,7 @@
   describe command('curl -L localhost:8153/go') do
       its(:stdout) { should contain('/go/home') }
   end
-  describe command('curl -L http://localhost:8153/go/agents') do
+  describe command('curl -H \'Accept: application/vnd.go.cd.v4+json\' localhost:8153/go/api/agents') do
     its(:stdout) { should contain('/var/lib/go-agent') }
     its(:stdout) { should contain('Idle') }
   end