Introduce a library of Jinja2 macros that will be handy to use in
templates. For now we add only 2 macros:

    cfg(name)   - to get instance configuration parameter `name`,   and
    cfg_bool    - to get truth value of ----//----

The reason we introduce cfg() is that we will need to use a lot of
parameters in many places and it is much more handy to write, e.g.

    cfg('email_enabled')

compared to

    instance_parameter:configuration.email_enabled

/cc @kazuhiko, @jerome