File tasks.yaml made optional for V4 plugins

Plugin developers don't need to have tasks.yaml any more.

Change-Id: Ieff19c5a1d8f9a647076afdf2b4028e96be1ce7e
Closes-Bug: #1552248

fuel_plugin_builder/templates/v4/plugin_data/tasks.yaml

@@ -1,5 +1,5 @@
 # WARNING: `tasks.yaml` will be deprecated in further releases.
-# Please, use `deployment_tasks.yaml` to describe tasks istead.
+# Please, use `deployment_tasks.yaml` to describe tasks instead.
 
 # This tasks will be applied on controller nodes,
 # here you can also specify several roles, for example

fuel_plugin_builder/tests/test_validator_v4.py

@@ -424,3 +424,11 @@ class TestValidatorV4(TestValidatorV3):
     @mock.patch('fuel_plugin_builder.validators.validator_v4.utils')
     def test_check_tasks_schema_validation_passed(self, utils_mock, *args):
         pass
+
+    @mock.patch('fuel_plugin_builder.validators.base.utils.exists')
+    def test_check_tasks_schema_validation_no_file(self, exists_mock, *args):
+        mocked_methods = ['validate_schema']
+        self.mock_methods(self.validator, mocked_methods)
+        exists_mock.return_value = False
+        self.validator.check_tasks_schema()
+        self.assertFalse(self.validator.validate_schema.called)

fuel_plugin_builder/validators/validator_v4.py

@@ -47,6 +47,7 @@ class ValidatorV4(ValidatorV3):
         self.validate_file_by_schema(
             self.schema.tasks_schema,
             self.tasks_path,
+            allow_not_exists=True,
             allow_empty=True
         )