Consolidate boolean/mk_boolean conversion functions into a single location

Consolidate the module_utils, constants, and config functions that
convert values into booleans into a single function in module_utils.

Port code to use the module_utils.validate.convert_bool.boolean function
isntead of mk_boolean.

lib/ansible/executor/play_iterator.py

@@ -23,14 +23,11 @@ import fnmatch
 
 from ansible import constants as C
 from ansible.module_utils.six import iteritems
+from ansible.module_utils.parsing.convert_bool import boolean
 from ansible.playbook.block import Block
 from ansible.playbook.task import Task
 
 
-boolean = C.mk_boolean
-
-__all__ = ['PlayIterator']
-
 try:
     from __main__ import display
 except ImportError:
@@ -38,6 +35,9 @@ except ImportError:
     display = Display()
 
 
+__all__ = ['PlayIterator']
+
+
 class HostState:
     def __init__(self, blocks):
         self._blocks = blocks[:]
@@ -300,10 +300,10 @@ class PlayIterator:
                     # NOT explicitly set gather_facts to False.
 
                     gathering = C.DEFAULT_GATHERING
-                    implied = self._play.gather_facts is None or boolean(self._play.gather_facts)
+                    implied = self._play.gather_facts is None or boolean(self._play.gather_facts, strict=False)
 
                     if (gathering == 'implicit' and implied) or \
-                       (gathering == 'explicit' and boolean(self._play.gather_facts)) or \
+                       (gathering == 'explicit' and boolean(self._play.gather_facts, strict=False)) or \
                        (gathering == 'smart' and implied and not (self._variable_manager._fact_cache.get(host.name, {}).get('module_setup', False))):
                         # The setup block is always self._blocks[0], as we inject it
                         # during the play compilation in __init__ above.