Add ability to use a non-admin controller-local user

juju-deployer, by way of jujuclient, can auth to a controller/model using either the local admin user (via user/pass auth), or using a USSO user, provided that user already has access to the model on the host on which deployer is running (and by access, we mean that they have an active, discharged macaroon in ~/.go-cookies).

What juju-deployer can not do presently is connect as a non-admin controller-local user, i.e. a user that was created with `juju add-user`. To connect as such a user, the username and password would need to be explicitly passed to juju-deployer, since they are not available on the filesystem (like the admin password is). These credentials would then need to be passed through to python-jujuclient.