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/plugins/lookup/first_found.py

@@ -122,9 +122,9 @@ import os
 
 from jinja2.exceptions import UndefinedError
 
-from ansible.constants import mk_boolean as boolean
 from ansible.errors import AnsibleFileNotFound, AnsibleLookupError, AnsibleUndefinedVariable
 from ansible.module_utils.six import string_types
+from ansible.module_utils.parsing.convert_bool import boolean
 from ansible.plugins.lookup import LookupBase
 
 
@@ -145,7 +145,7 @@ class LookupModule(LookupBase):
                 if isinstance(term, dict):
                     files = term.get('files', [])
                     paths = term.get('paths', [])
-                    skip = boolean(term.get('skip', False))
+                    skip = boolean(term.get('skip', False), strict=False)
 
                     filelist = files
                     if isinstance(files, string_types):

lib/ansible/plugins/lookup/hashi_vault.py

@@ -46,8 +46,8 @@ __metaclass__ = type
 import os
 
 from ansible.errors import AnsibleError
+from ansible.module_utils.parsing.convert_bool import boolean
 from ansible.plugins.lookup import LookupBase
-from ansible.constants import mk_boolean as boolean
 
 HAS_HVAC = False
 try:
@@ -149,7 +149,7 @@ class HashiVault:
         self.client.auth_ldap(username, password, mount_point)
 
     def boolean_or_cacert(self, validate_certs, cacert):
-        validate_certs = boolean(validate_certs)
+        validate_certs = boolean(validate_certs, strict=False)
         '''' return a bool or cacert '''
         if validate_certs is True:
             if cacert != '':

lib/ansible/plugins/lookup/subelements.py

@@ -17,9 +17,9 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
-from ansible.constants import mk_boolean as boolean
 from ansible.errors import AnsibleError
 from ansible.module_utils.six import string_types
+from ansible.module_utils.parsing.convert_bool import boolean
 from ansible.plugins.lookup import LookupBase
 from ansible.utils.listify import listify_lookup_plugin_terms
 
@@ -72,7 +72,7 @@ class LookupModule(LookupBase):
                 # this particular item is to be skipped
                 continue
 
-            skip_missing = boolean(flags.get('skip_missing', False))
+            skip_missing = boolean(flags.get('skip_missing', False), strict=False)
             subvalue = item0
             lastsubkey = False
             sublist = []