bug#61500: Default cache directory of cat-avatar-generator-service

[Evgeny Pisemsky <evgeny@pisemsky.com>]

Hello!

The cache directory must:

1. End with a slash (due to implementation of the php script).
2. Exist or be created manually.
3. Be writeable by php-fpm.

The current default value doesn't meet these requirements and leads to
broken caching and errors in php-fpm log.

The simplest solution is to change it to /tmp/, but for obvious
reasons it is not suitable for production use.

However, since this service is described as demonstration, I would
prefer unsafe default over broken (for now).

Also the requirements above should probably be mentioned in the
documentation of the service.