Exclude private subnets when provisioning in AWS

[peterschmidt85]

Currently, dstack uses the first subnet it finds. If the VPC contains private subnets, this causes provisioning logic to fail and the run to hang while attempting to connect via SSH.:

DEBUG 2024-02-14T13:28:09.616 dstack._internal.core.services.ssh.tunnel SSH tunnel failed: b'ssh: connect to host 34.229.201.59 port 22: Operation timed out\r\n'

[peterschmidt85]

@r4victor Private subnets should be excluded even if the VPC is not specified in ~/.dstack/server/config.yml.

Currently, if the default VPC has private subnets and the VPC name is not specified, runs will fail.

[peterschmidt85]

Should be fixed in 0.15.2rc2