support templates only in values like ansible playbooks

[ssato]

it should be useful if some values are jinja2 template string instead of entire configuration files are jinja2 templates.

[ssato]

It seems difficult or not difficult but farely costly than I expected. Maybe I can extend parse_value() to compile template strings but it needs to be implemented in all backend parsers because only xml and ini backend parse values if ac_parse_value option was given, or all value template strings should be compiled after nested dict contains them is constructed.