Replace subprocess with python-libjuju calls

[Dariusz Smigiel]

There are few places in the code, which rely on running `subprocess` and using `juju`.

It needs to be replaced with appropriate `python-libjuju` calls.

[Zachary Zehring (zzehring)]

Currently, python-libjuju does not implement a couple of key functions we need for the tool: retrieving all controllers and creating controller backups.

Here are the GH Issues for implementing those features:

controllers: https://github.com/juju/python-libjuju/issues/529
controller backup: https://github.com/juju/python-libjuju/issues/529

Once those are implemented, we can remove the subprocess calls and add the lib calls.

[Jose Guedez (jfguedez)]

The upstream issues have been resolved. The current code in master uses the API for creating controller backups.