Add config option to allow specifying juju version

juju-local charm doesn't currently provide the option to specify juju snap channels (https://git.launchpad.net/charm-juju-local/tree/reactive/charm_juju_local.py#n26) for installation. As the default channel of juju has been changed to 3.1/stable but 2.9/stable is still widely used in practice, it is necessary to give users the flexibility choose a juju version fitting their need.

I propose adding a config option called `juju-version` or `juju-channel` to accept user's input and install correct version of juju snap accordingly.