CLI: get rid of cheetah for output generation

[Salvatore Orlando]

We currently generate CLI output using Cheetah templates.
While this allowed us to quickly implement nicer output and unit tests, there are several arguments against using Cheetah to this aim:

1) A new dependency always creates problems. We witnessed some of them with the Jenkins failures. The less dependencies a software component has, the better it is.
2) Cheetah is a bit of an overshoot for our needs. We don't need to generate complex documents, but just a few output lines.
3) The CLI loads the template file every time a command is executed; this is not very efficient.

We should therefore consider replacing Cheetah with a mechanism based on Python built-in templating capabilities.
We would probably need support for iterations (see list operations), and structured data (for printing data for network and ports).