Remove the automatic configuration of AWS/GCP/Azure/OCI backends

[peterschmidt85]

Currently, we have to ways to configure backends:

1. Automatic - dstack server automatically detects credentials and creates ~/.dstack/server/config.yml
2. Explicit - when the user manually create ~/.dstack/server/config.yml

Problems:

1. Maintaining two ways is hard also because it requires documentation and explaining when to use which.
2. The automatic configuration doesn't allow configuring many properties such as VPC, etc. More often than not, it's better if the user configures it explicitely.
3. The automatic configuration works only for a subset of providers.

Solution: Remove the automatic configuration and always expect the explicit configuration from the user