Auth/AD: make PVE::Auth::AD a subclass of PVE::Auth::LDAP

this makes it much easier to reuse the sync code from LDAP in AD.
The 'authenticate_user' sub is still the same, but we now
can still use the get_users and get_groups functionality of LDAP

in the case of AD, the user_attr is optional in the config
(would have been a breaking change) but we set it
to default to 'sAMAccountName'

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>