addc: support alternate method for supplying the admin password

[phlogistonjohn]

Currently, the only place sambacc sources the admin password from is in the sambacc config file. This means if you want to keep the password more secure you have to put the entire config file in a secret (or equivalent). This is inconvient if you want to keep your sambacc configurations together in one place.

We could have sambacc source the password from another file and/or an environment variable. Off the top of my head I know that both podman and k8s have methods for passing secret data via those means.

This is not an issue for domain member servers because those already take the join user/pass in a separate file from the configuration.