Allow multiple values in `amazonec2-instance-type`

When configure auto scaling on AWS you may specify to use spot instances and the config value amazonec2-instance-type says what instance type.

A few times every month my spot requests fails due some reasons. Most common are "Capacity not available". My current workaround is to change the Gitlab runner config to another instance and then restart.

A possible solution would be that I specify multiple instance types in amazonec2-instance-type and if the first fail move on to another. I don't really care if I get one instance or another but I do care if I get none at all.