names: Use hostnamectl instead of socket API get the current hostname

- This would mean consistent interfacing with systemd instead of a split
approach.

Tests:

- Edit hostname page shows the current value of hostname correctly.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/names/__init__.py

@@ -242,7 +242,9 @@ def get_domain_name():
 
 def get_hostname():
     """Return the hostname."""
-    return socket.gethostname()
+    process = subprocess.run(['hostnamectl', 'hostname', '--static'],
+                             stdout=subprocess.PIPE, check=True)
+    return process.stdout.decode().strip()
 
 
 def set_hostname(hostname):