Avoid crash in check mode (#243)

* Do not let AnsibleModule crash when setting permissions on not yet existing files in check mode.

* Add tests.

* Fix bugs.

plugins/module_utils/openssh/backends/keypair_backend.py

@@ -207,6 +207,8 @@ class KeypairBackend(object):
         file_args = self.module.load_file_common_arguments(self.module.params)
         if public_key:
             file_args['path'] = file_args['path'] + '.pub'
+        if self.module.check_file_absent_if_check_mode(file_args['path']):
+            return True
         return self.module.set_fs_attributes_if_different(file_args, False)
 
     @property