Properly scope shared variable spaces

• Supersede enforced configuration of kerberized NFS servers using (non-shared) special host groups.
• Scope REALM and smb_domain variables to their respective roles. They can be automatically generated anyway.

ad-server/defaults/main.yml

@@ -3,3 +3,5 @@
 ad_admin_password: samba-admin
 smb_model_prefork: false
 smb_prefork_children: 4  # since it is default in 4.10
+smb_domain: "{{ domain.split('.')[0].upper() }}"
+REALM: "{{ domain.upper() }}"