Set file as the default template handler type (#1127)

Now that we have template handlers we want to encourage users to
migrate from using `template_path` to `template` in their Sceptre
stack config files.  To make that transition easier we set the
default `template` type to 'file' which will save the user from
having to specify a full `template` block if they just want to
load a file from disk

docs/_source/docs/template_handlers.rst

@@ -28,6 +28,8 @@ file
 Loads a template from disk. Supports JSON, YAML, Jinja2 and Python files. Will be used if the ``template_path`` Stack
 config property is set, for backwards compatibility reasons.
 
+This is the default template handler type, setting the ``file`` type is option.
+
 Syntax:
 
 .. code-block:: yaml
@@ -41,7 +43,6 @@ Example:
 .. code-block:: yaml
 
    template:
-     type: file
      path: storage/bucket.yaml
 
 s3

sceptre/template.py

@@ -56,6 +56,8 @@ def __init__(
 
         self.name = name
         self.handler_config = handler_config
+        if self.handler_config is not None and self.handler_config.get('type') is None:
+            self.handler_config['type'] = 'file'
         self.sceptre_user_data = sceptre_user_data
         self.stack_group_config = stack_group_config
         self.connection_manager = connection_manager

tests/test_template.py

@@ -48,6 +48,17 @@ def setup_method(self, test_method):
             connection_manager=connection_manager,
         )
 
+    def test_initialise_template_default_handler_type(self):
+        template = Template(
+            name="template_name",
+            handler_config={"path": "/folder/template.py"},
+            sceptre_user_data={},
+            stack_group_config={},
+            connection_manager={},
+        )
+
+        assert template.handler_config == {"type": "file", "path": "/folder/template.py"}
+
     def test_initialise_template(self):
         assert self.template.handler_config == {"type": "file", "path": "/folder/template.py"}
         assert self.template.name == "template_name"