config: Handle totem_set_volatile_defaults errors

When totem_set_volatile_defaults is called from totem_config_validate
return code is unchecked.

It's then perfectly possible to set (for example) join timeout to very
small value (1) and consensus value is then set to 0 making corosync
unable to create membership.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>