LDAP: allow multiple user principals
    
    In general a user can have multiple principals and recent IPA version
    added support to defined multiple principals. With this patch SSSD does
    not only store the first but all principals read by LDAP from a server.
    
    Resolves https://fedorahosted.org/sssd/ticket/2958
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>