rhn_register fixes for username and password when unregistering

- Require username and password for unregistering and avoid "cannot marshal None unless allow_none is enabled" error when using an activation key and no channels specified.
- Update test fixtures and add changelog


Co-authored-by: WhyIsThisOpen <WhyIsThisOpen@users.noreply.github.com>

test/units/modules/packaging/os/test_rhn_register.py

@@ -110,7 +110,7 @@ def test_without_required_parameters(capfd, patch_rhn):
 TESTED_MODULE = rhn_register.__name__
 TEST_CASES = [
     [
-        # Registering an unregistered host and subscribing to one channel
+        # Registering an unregistered host with channels
         {
             'channels': 'rhel-x86_64-server-6',
             'username': 'user',
@@ -137,9 +137,11 @@ TEST_CASES = [
         }
     ],
     [
-        # Registering an unregistered host with only an activationkey and without subscribing any channels
+        # Registering an unregistered host without channels
         {
             'activationkey': 'key',
+            'username': 'user',
+            'password': 'pass',
         },
         {
             'calls': [