Fix apt-get update retry failed when apt-get update failed during add-topology (sonic-net#6677)

What is the motivation for this PR?
If apt-get update failed, there was no attribute cache_updated in returned value, so the retry condition would not execute.

How did you do it?
Add judgment for cache_updated, if it was defined, then check its value.

Signed-off-by: Chun'ang Li <chunangli@microsoft.com>

Author: lerry-lee

ansible/roles/vm_set/tasks/docker.yml

@@ -54,7 +54,7 @@
   become: yes
   environment: "{{ proxy_env | default({}) }}"
   register: apt_update_res
-  until: apt_update_res.cache_updated
+  until: apt_update_res.cache_updated is defined and apt_update_res.cache_updated
   retries: 5
   delay: 10