support template rendering when file attribute is set

[httpdss]

When defining a file attrubute the template renderer should be called so that the content of that file is processed by the rendering class.

current behaviour

if this is set on the struct file, then content is just copied down to the file content

files:
  - myfile.txt:
      file: /path/to/file.txt

expected behaviour

/path/to/file.txt should be passed to the template renderer so that it gets processed