engine : autoscaling pass instance id's not names to loadbalancer

Regression caused by 4bc1bcb608379cc396113a03f2ae37ce78f9049c

We need to pass the id_list to the loadbalancer, otherwise it
can't lookup the instance IP adresses with nova and the loadbalancer
config ends up with 0.0.0.0 for every instance

Fixes bug #1192128

Change-Id: I73bab70e897c0fb0b317793fa31ff8d07720a6c4
Merge "engine : autoscaling pass instance id's not names to loadbalancer"