jinja2_template.py: open 'once' file in 'ab' mode

This is to avoid overwriting the contents of the file specified by the
'once' option, which makes it possible to use the rendered file as the
'once' file.

slapos/recipe/template/README.jinja2.txt

@@ -555,3 +555,40 @@ Removing the canary allows template to be re-rendered::
     >>> cat('foo_once')
     dummy
 
+It's also possible to use the same file for ``rendered`` and ``once``::
+
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts = template
+    ...
+    ... [template]
+    ... recipe = slapos.recipe.template:jinja2
+    ... template = inline:initial content
+    ... rendered = rendered
+    ... once = ${:rendered}
+    ... ''')
+    >>> run_buildout() # doctest: +ELLIPSIS
+    Uninstalling template.
+    Installing template.
+    The template install returned None.  A path or iterable os paths should be returned.
+
+Template was rendered::
+
+    >>> cat('rendered')
+    initial content
+
+When buildout options are modified, the template will not be rendered again::
+
+    >>> with open('buildout.cfg', 'a') as f:
+    ...     f.writelines(['template = inline:something different'])
+
+    >>> run_buildout()
+    Uninstalling template.
+    Installing template.
+    The template install returned None.  A path or iterable os paths should be returned.
+
+Even though we used a different template, the file still contain the first template::
+
+    >>> cat('rendered')
+    initial content

slapos/recipe/template/jinja2_template.py

@@ -266,7 +266,7 @@ class Recipe(object):
         finally:
             os.close(fd)
         if self.once:
-            open(self.once, 'wb').close()
+            open(self.once, 'ab').close()
             return
         return self.rendered