vmware_category/test: ensure test can run on Zuul

- set the zuul/vmware/vcenter_only alias
- import `prepare_vmware_tests` role
- little refactoring to avoid an extra level of indentation

test/integration/targets/vmware_category/aliases

@@ -1,3 +1,4 @@
 shippable/vcenter/group1
 cloud/vcenter
 needs/target/prepare_vmware_tests
+zuul/vmware/vcenter_only

test/integration/targets/vmware_category/tasks/associable_obj_types.yml

@@ -2,8 +2,6 @@
 # Copyright: (c) 2019, Abhijeet Kasurde <akasurde@redhat.com>
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
-- when: vcsim is not defined
-  block:
 - name: Create different types of category with associable object types
   vmware_category:
     hostname: '{{ vcenter_hostname }}'

test/integration/targets/vmware_category/tasks/main.yml

@@ -2,4 +2,8 @@
 # Copyright: (c) 2019, Abhijeet Kasurde <akasurde@redhat.com>
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
+- when: vcsim is not defined
+  block:
+  - import_role:
+      name: prepare_vmware_tests
   - include: associable_obj_types.yml