Autoscaling: settings for AMI and Custom boot script shall be linked and made into a list

[sanderegg]

Use-case: in a computational cluster I send jobs of different kind.

On the computational cluster manager we have:

• dask-scheduler
• autoscaling

When sending jobs there, we will probably pass as resource the EC2 instance type that we want.

For each type, we might want:

• a different AMI
• a different boot script

One way of allowing this, is to make the autoscaling service there have a list of entries:
EC2_INSTANCES_ALLOWED_TYPES would become an object with:

EC2_INSTANCES_ALLOWED_TYPES
EC2_INSTANCES_ALLOWED_AMI_ID
EC2_INSTANCES_CUSTOM_BOOT_SCRIPTS

At least these ENVs should be linked together.

[sanderegg]

fixed by #5033