iSCSITarget: properly create portals for lio-t implementation

Even when defining the "portals" parameter, targetcli creates a
default portal at 0.0.0.0:3260 if the auto_add_default_portal option
is set to true (default), causing the resource to fail with
OCF_ERR_GENERIC.

Suppress creation of the default portal by setting
auto_add_default_portal=false if the "portals" parameter is set,
and retain the original behavior if it is not.

Fixes ClusterLabs/resource-agents#630.