Allow ApplicationFacade set_config with non-string values

[cderici]

Description

So we want to be able to do the following:

    config = await application.get_config()
    await application.set_config(config)

However, the issue was, that on the Juju side while the get_config returns a map[string]value, the set_config expects a map[string]string. This patch fixes it by casting everything into str on pylibjuju side before it's passed into the api.

Fixes #388

QA Steps

There's a test included in the patch that targets this, so the following should pass:

 $ tox -e integration -- tests/integration/test_application.py::test_get_set_config

Notes & Discussion

So clearly this is not the ideal solution, which would be to have Juju accept configs that look like map[string]value and cast things individually as it sees fit. However, @SimonRichardson suggests that that has the potential of breaking a lot of things, so looks like we'll leave this as a future work.

Note that this also means that any library (other than pylibjuju) that wants to use Set has to do the same thing on their own, or abandon their project and start using pylibjuju instead, which would be the cooler option.

[SimonRichardson]

$$merge$$

[SimonRichardson]

Can you rebase against master and we'll try again.

[cderici]

!!build!!

[cderici]

$$merge$$