migration: Use api.env variables.
    
    Use api.env.basedn instead of anonymously accessing LDAP to get base DN.
    Use api.env.basedn instead of searching filesystem for ldapi socket.
    
    https://fedorahosted.org/freeipa/ticket/4953
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>