Avoid facts gathered twice when deploying with --limit

When deploying with argument --limit, both first and second plays
in site.yml will be processed, causing host facts gathered twice,
which is unnecessary. Thus a condition is added to the first play
to avoid such redundancy.

Change-Id: I20b84c59bf73aaeca929fc56465ff2cf1cf52b8e
Closes-Bug: #1691033

ansible/site.yml

@@ -10,8 +10,9 @@
   gather_facts: false
   tasks:
     - setup:
-  tags: always
-
+      when:
+        - (play_hosts | length) == (groups['all'] | length)
+  
 # NOTE(pbourke): This case covers deploying subsets of hosts using --limit. The
 # limit arg will cause the first play to gather facts only about that node,
 # meaning facts such as IP addresses for rabbitmq nodes etc. will be undefined