address a few follow-up issues for Org -> Galaxy Credentials support

- add support for managing galaxy creds in the tower organization module
- fix a minor serializer bug

awx_collection/tests/integration/targets/tower_organization/tasks/main.yml

@@ -12,6 +12,8 @@
 - name: "Create a new organization"
   tower_organization:
     name: "{{ org_name }}"
+    galaxy_credentials:
+      - Ansible Galaxy
   register: result
 
 - assert: