Wait for idle arg type check

[cderici]

Description

Currently there's no check for the apps argument for the wait_for_idle() method. So it goes through any iterable that's passed to it, including strings. As a result wait_for_idle(apps="test") produces something like the following:

INFO     juju.model:model.py:2645 Waiting for model:
  t (missing)
  e (missing)
  s (missing)
  t (missing)

This introduces a check for the argument to be a List[str].

Fixes #732

QA Steps

Unit tests are added, should be a simple QA.

tox -e py3 -- tests/unit/test_model.py::TestModelWaitForIdle

[juanmanuel-tirado]

Apart from the problems regarding the testing phase, this LG2M

[cderici]

Apart from the problems regarding the testing phase, this LG2M

This CI issue is slightly more than trivial so it'll be fixed in a new seperate PR, already up at #742 .

[cderici]

/merge