Plan moving ALBs to their own subnet

[bengerman13]

Currently, our ALBs are in the same subnets as bosh deployments. This means that it is possible for an ALB to take up an IP address that bosh wants to use for a scale-out. Bosh then fails with no straightforward workaround.
This has prevented scale-outs from working on multiple occasions.

We should move ALBs into a different, dedicated subnet, and ensure they still have access to the resources required.

there are a few ways we can go about this. I think the easiest and safest looks like this:

• create a new subnet for ALBs
• create the routing rules required
• create new ALBs that are exact copies, except they exist in the new subnets
• test the new ALBs manually
• update relevant DNS entries. Where possible, these should include both the new and the old ALBs
• wait some time
• Update relevant DNS entries again, removing old ALBs
• wait some time
• remove old ALBs

Acceptance criteria:

• list of ALBs to be moved
• planned block for new subnet
• plan for moving ALBs (I believe there will be some ALBs that are more difficult to move and some that are easier. There may even be some that do not require moving)

Extra credit:

• can we upgrade all our old ELBs at the same time