Add support for become_flags on the task level (#17310)

Working on the test suite, I tried to replace a call to sudo to a
call to su, and found out that I can't change user to 'nobody'
without changing the option become_flags in ansible.cfg

As this would be dependent on the user and the task, it make more sense
to push the setting there.

lib/ansible/playbook/become.py

@@ -35,6 +35,7 @@ class Become:
     _become              = FieldAttribute(isa='bool')
     _become_method       = FieldAttribute(isa='string')
     _become_user         = FieldAttribute(isa='string')
+    _become_flags        = FieldAttribute(isa='string')
 
     def __init__(self):
         return super(Become, self).__init__()

lib/ansible/playbook/play_context.py

@@ -122,6 +122,7 @@ TASK_ATTRIBUTE_OVERRIDES = (
     'become_user',
     'become_pass',
     'become_method',
+    'become_flags',
     'connection',
     'docker_extra_args',
     'delegate_to',