Very similar to JENKINS-28157

I changed the update-sites to update-center.json but it still tries to download plugins from 
updates.jenkins-ci.org.

```

Caused by: java.net.UnknownHostException: updates.jenkins-ci.org
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1944)
at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1939)
at java.security.AccessController.doPrivileged(Native Method)
at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1938)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1508)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492)
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1181)
... 7 more
Caused by: java.net.UnknownHostException: updates.jenkins-ci.org
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)
at sun.net.www.http.HttpClient.<init>(HttpClient.java:242)
at sun.net.www.http.HttpClient.New(HttpClient.java:339)
at sun.net.www.http.HttpClient.New(HttpClient.java:357)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1220)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1156)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1050)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:984)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1564)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492)
at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:3000)
at java.net.URLConnection.getHeaderFieldLong(URLConnection.java:629)
at java.net.URLConnection.getContentLengthLong(URLConnection.java:501)
at java.net.URLConnection.getContentLength(URLConnection.java:485)
at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1165)
... 7 more
```