Many: expose rhsm.subscription_manager.rhsm.auto_enable_yum_plugins

Expose the ${SUBJ} customization in Cloud API. Weldr API inherited it
automatically with the update of `osbuild/blueprint` to v1.6.0.

Adjust relevant test cases and unit tests to use this new customization.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

test/cases/api.sh

@@ -472,7 +472,8 @@ RHSM_CUSTOMIZATION_BLOCK=$(cat <<EOF
         },
         "subscription_manager": {
           "rhsm": {
-            "manage_repos": true
+            "manage_repos": true,
+            "auto_enable_yum_plugins": false
           },
           "rhsmcertd": {
             "auto_registration": false