roles/samba-dc: Support joining existing domain

The *samba-dc* role now supports joining an existing Active Directory
domain as an additional domain controller. The `samba_is_first_dc` variable
controls whether the machine will be provisioned with a new domain (when
true) or added to an existing domain (when false).

Joining an existing domain naturally requires credentials of a user with
permission to add a new DC, the `samba_dc_join_username` and
`samba_dc_join_password` variables can be used to specify them.
Alternatively, if these variables are not defined, then the process will
attempt to use Kerberos credentials. This would require playbooks to
make a ticket-granting-ticket available somehow, such as by executing
`kinit` prior to applying the *samba-dc* role.

roles/samba-dc/tasks/main.yml

@@ -32,6 +32,9 @@
     domain={{ netbios_domain|d(omit) }}
     use_rfc2307={{ samba_dc_use_rfc2307 }}
     dns_backend={{ samba_dc_dns_backend|d(omit) }}
+    username={{ samba_dc_join_username|d(omit) }}
+    password={{ samba_dc_join_password|d(omit) }}
+    state={{ 'provisioned' if samba_is_first_dc else 'joined' }}
   register: samba_dc_provision
   notify:
   - restore samba file contexts