Remove shell module specialcases

Shell is implemented via the command module.  There was a special case
in mod_args to do that.  Make shell into an action plugin to handle that
instead.

Also move the special case for the command nanny into a command module
action plugin.  This is more appropriate as we then do not have to send
a parameter that is only for the command module to every single module.

lib/ansible/plugins/action/__init__.py

@@ -612,9 +612,6 @@ class ActionBase(with_metaclass(ABCMeta, object)):
         # make sure all commands use the designated shell executable
         module_args['_ansible_shell_executable'] = self._play_context.executable
 
-        # let command know it should avoid specific warnings
-        module_args['_ansible_command_warnings'] = C.COMMAND_WARNINGS
-
     def _update_connection_options(self, options, variables=None):
         ''' ensures connections have the appropriate information '''
         update = {}