Platform should use a well-known domain for device-type discovery

[dfunckt]

The platform looks up available device types in the production S3 bucket of balenaCloud. We're currently pointing instances directly to the bucket, but we should instead use files.balena-cloud.com which proxies that bucket. The two obvious benefits are that users would then be able to deploy openbalena in strict networks (where all outbound connections must be explicitly allowed by a firewall and whitelisting s3.amazonaws.com is too broad) and also allows balenaCloud to change the actual storage location.

[balena-ci]

[roman-mazur] This issue has attached support thread https://jel.ly.fish/#/support-thread~c0c4609d-1416-4308-be00-a9d81b313ad6