Skip generating dualtor_facts on single tor testbed. (#2646)

I noticed that the dual_tor_facts plugin costs around 5 minutes to
generate dualtor facts when deploying or generating minigraph.
The generated dualtor facts is not used on single tor testbed though.
This commit updates the task file for deploying minigraph to skip
dual_tor_facts plugin on single tor testbed.

Signed-off-by: bingwang <[email protected]>

Author: bingwang-ms

ansible/config_sonic_basedon_testbed.yml

@@ -65,9 +65,15 @@
   - topo_facts: topo={{ topo }}
     delegate_to: localhost
 
+  - name: set default dualtor facts
+    set_fact:
+      dual_tor_facts: {}
+    when: "'dualtor' not in topo"
+    
   - name: gather dual ToR information
     dual_tor_facts: hostname="{{ inventory_hostname }}" testbed_facts="{{ testbed_facts }}" hostvars="{{ hostvars }}" vm_config="{{ vm_topo_config }}"
     delegate_to: localhost
+    when: "'dualtor' in topo"
 
   - name: set default vm file path
     set_fact: