Use IPAdmin rather than raw python-ldap in migration.py and ipadiscovery.py
    
    These used ipautil.get_ipa_basedn. Convert that to use the new wrappers.
    
    Beef up the error handling in ipaldap to accomodate the errors we catch
    in the server discovery.
    Add a DatabaseTimeout exception to errors.py.
    
    These were the last uses of ipautil.convert_ldap_error, remove that.
    
    https://fedorahosted.org/freeipa/ticket/3487
    https://fedorahosted.org/freeipa/ticket/3446