add user:password

This usage pattern, in my opinion, is very bad. It forces setting a secret in inventory directly, which is a bad practice - inventory files are private, but meant to contain only regular information. `debops.secret` role can manage confidental information like passwords securely and automatically.

However, automatically generating passwords here using something like
```
users_username:
  - name: 'user'
    password: '{{ secret + "/credentials/" + ansible_fqdn + "/users/" + ... + "/password" }}'
```
doesn't work because Ansible won't handle nested hash values... Honestly, `debops.users` is meant for a very small number of accounts, and better LDAP integration is coming along soon, so maybe I'll stop worrying about it. However, I would add a note about discouraging this use.