Use config-yaml by default when deploying a bundle.
Use config-yaml by default when deploying a bundle.

This invokes better/more modern behavior for blank strings in the
options for a given charm in the bundle.yaml. Specifically, it allows us
to deploy landscap-dense-maas without running into a KeyError.

This has been tested by using matrix to successfully deploy
landscape-dense-maas, using the code in this PR.

juju/python-libjuju,juju/python-libjuju