gather facts improvements (#44008)

* gather_subset is a list of strings

When gather_subset is an integer, a message pointing out the problem, current tb error 
is replaced by this one:

    ERROR! the field 'gather_subset' should be a list of (<class 'str'>,), but the item '42' is a <class 'int'>
...
* gathering_facts test: ensure smart gathering is on

lib/ansible/config/base.yml

@@ -589,7 +589,7 @@ DEFAULT_GATHERING:
   choices: ['smart', 'explicit', 'implicit']
 DEFAULT_GATHER_SUBSET:
   name: Gather facts subset
-  default: 'all'
+  default: ['all']
   description:
       - Set the `gather_subset` option for the M(setup) task in the implicit fact gathering.
         See the module documentation for specifics.

lib/ansible/playbook/play.py

@@ -59,7 +59,7 @@ class Play(Base, Taggable, Become):
     # Facts
     _fact_path = FieldAttribute(isa='string', default=None)
     _gather_facts = FieldAttribute(isa='bool', default=None, always_post_validate=True)
-    _gather_subset = FieldAttribute(isa='list', default=None, always_post_validate=True)
+    _gather_subset = FieldAttribute(isa='list', default=None, listof=string_types, always_post_validate=True)
     _gather_timeout = FieldAttribute(isa='int', default=None, always_post_validate=True)
 
     # Variable Attributes

test/integration/targets/gathering_facts/test_run_once.yml

@@ -2,6 +2,10 @@
 - hosts: facthost1
   gather_facts: no
   tasks:
+    - name: check that smart gathering is enabled
+      fail:
+        msg: 'smart gathering must be enabled'
+      when: 'lookup("env", "ANSIBLE_GATHERING") != "smart"'
     - name: install test local facts
       copy:
         src: uuid.fact