React to hostname changes

Closes #94

src/app.jsx

@@ -28,8 +28,7 @@ export class Application extends React.Component {
         super();
         this.state = { 'hostname': _("Unknown") };
 
-        cockpit.file('/etc/hostname').read()
-                .done((content) => {
+        cockpit.file('/etc/hostname').watch(content => {
             this.setState({ 'hostname': content.trim() });
         });
     }

test/check-application

@@ -28,6 +28,11 @@ class TestApplication(testlib.MachineCase):
         b.wait_present(".container-fluid p")
         b.wait_text(".container-fluid p", "Running on " + hostname)
 
+        # change current hostname
+        m.execute("echo new-%s > /etc/hostname" % hostname)
+        # verify new hostname name
+        b.wait_text(".container-fluid p", "Running on new-" + hostname)
+
         # change language to German
         b.switch_to_top()
         b.click("#content-user-name")