Currently slapos.recipe.template:jinja2 fails if source contains
non-ascii input:

    <BLANKLINE>
    An internal error occurred due to a bug in either zc.buildout or in a
    recipe being used:
    Traceback (most recent call last):
      File ".../site-packages/zc/buildout/buildout.py", line 1992, in main
        getattr(buildout, command)(args)
      File ".../site-packages/zc/buildout/buildout.py", line 550, in install
        [self[part]['recipe'] for part in install_parts]
      File ".../site-packages/zc/buildout/buildout.py", line 1135, in __getitem__
        options._initialize()
      File ".../site-packages/zc/buildout/buildout.py", line 1239, in _initialize
        self.initialize()
      File ".../site-packages/zc/buildout/buildout.py", line 1248, in initialize
        self.recipe = recipe_class(buildout, name, self)
      File ".../slapos.recipe.template/slapos/recipe/template/jinja2_template.py", line 208, in __init__
        env.compile(source, filename=template),
      File ".../site-packages/jinja2/environment.py", line 551, in compile
        source = self._parse(source, name, filename)
      File ".../site-packages/jinja2/environment.py", line 470, in _parse
        return Parser(self, source, name, encode_filename(filename)).parse()
      File ".../site-packages/jinja2/parser.py", line 31, in __init__
        self.stream = environment._tokenize(source, name, filename, state)
      File ".../site-packages/jinja2/environment.py", line 501, in _tokenize
        source = self.preprocess(source, name, filename)
      File ".../site-packages/jinja2/environment.py", line 495, in preprocess
        self.iter_extensions(), text_type(source))
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 219: ordinal not in range(128)

it also fails writing-out non-ascii input (which could originate even
programmatically even if source was purely ascii):

    <BLANKLINE>
    An internal error occurred due to a bug in either zc.buildout or in a
    recipe being used:
    Traceback (most recent call last):
      File ".../site-packages/zc/buildout/buildout.py", line 1992, in main
        getattr(buildout, command)(args)
      File ".../site-packages/zc/buildout/buildout.py", line 666, in install
        installed_files = self[part]._call(recipe.install)
      File ".../site-packages/zc/buildout/buildout.py", line 1407, in _call
        return f()
      File ".../slapos/slapos.recipe.template/slapos/recipe/template/jinja2_template.py", line 227, in install
        out.write(self.template.render(**self.context))
    UnicodeEncodeError: 'ascii' codec can't encode characters in position 206-211: ordinal not in range(128)

Fix it.

@vpelletier suggested to add `encoding` option so it is flexible and
'utf-8' is not hardcoded.

/reviewed-by @vpelletier  (on !1)