email_server: Don't get domain name during initialization

This keeps the LE module working better when the domain name is changed after
startup.

Tests:

- Untested. The module is not enabled yet. LE functionality in email server is
not fully functional.

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

plinth/modules/email_server/__init__.py

@@ -82,17 +82,13 @@ class EmailServerApp(plinth.app.App):
         self.add(webserver)
 
         # Let's Encrypt event hook
-        default_domain = get_domainname()
-        domains = [default_domain] if default_domain else []
-        letsencrypt = LetsEncrypt('letsencrypt-email-server', domains=domains,
+        letsencrypt = LetsEncrypt('letsencrypt-email-server',
+                                  domains=get_domains,
                                   daemons=['postfix', 'dovecot'],
                                   should_copy_certificates=False,
                                   managing_app='email_server')
         self.add(letsencrypt)
 
-        if not domains:
-            logger.warning('Could not fetch the FreedomBox domain name!')
-
     def _add_ui_components(self):
         info = plinth.app.Info(
             app_id=self.app_id, version=version, name=self.app_name,
@@ -146,6 +142,12 @@ class EmailServerApp(plinth.app.App):
         return results
 
 
+def get_domains():
+    """Return the list of domains configured."""
+    default_domain = get_domainname()
+    return [default_domain] if default_domain else []
+
+
 def setup(helper, old_version=None):
     """Installs and configures module"""