Allow an overriding section to unset 'url' in order to instead define 'inline'

Example:

foo.cfg:

  [foo]
  url = ...

bar.cfg:

  [buildout]
  extends = foo.cfg

  [foo]
  url =
  inline = blablah

Recipes must not distinguish the existence of an option from an empty
option value. It's a common mistake in recipes because that's natural
to do that in Python. However, buildout has no syntax to remove an
option when overriding a section.

slapos/recipe/template/__init__.py

@@ -76,19 +76,17 @@ class Recipe(object):
 
     def _template(self, options):
         inline = options.get('inline')
-        try:
-            url = options['url']
-        except KeyError:
-            if inline is None:
-                raise
+        url = options.get('url')
+        if url:
+            if inline:
+                raise UserError("options 'inline' & 'url' conflict")
+            return False, url
+        if inline:
             if self.md5sum:
                 raise UserError("options 'inline' & 'md5sum' conflict")
             self.md5sum = True # tell update() to do nothing
             return True, inline
-        else:
-            if inline:
-                raise UserError("options 'inline' & 'url' conflict")
-            return False, url
+        raise UserError("one of the options 'inline' 'url' is required")
 
     def _init(self, name, options):
         self.output = options['output']