fixup! Clean up slapos.recipe.template ( f5ebda78 )

By changing jinja2-template users from overriding url= to defining
template= the original url= option was expanded, but this failed
because there is no filename=. Define a default filename to prevent
this.

This fixes errors like for example:

    While:
      Installing.
      Getting section instance.cfg.
      Initializing section instance.cfg.
      Getting option instance.cfg:url.
    Error: Referenced option does not exist: instance.cfg filename

when installing component/pygolang/test.cfg

stack/nxdtest.cfg

@@ -42,6 +42,7 @@ output = ${buildout:directory}/${:_buildout_section_name_}
 mode     = 0644
 context  =
   section buildout  buildout
+filename = inline
 
 [nxdtest-instance.cfg]
 <= jinja2-template