Support gateways without public IPs on AWS

[r4victor]

Closes #1171

This PR adds support for gateways without public IPs on AWS that can run services with https: false. The support for private gateways that can services with https: true is to be addressed in a separate issue.

The PR also:

• Makes AWS backend respect VPC configuration when creating gateway (default vpc/subnet was always used before).
• Fixes a bug introduced in Implement dstack apply for gateways #1223 (wrong gateway ssh key passed).

An example of a gateway without public IP configuration:

type: gateway
name: test-private-gateway
backend: aws
region: eu-central-1
domain: '*.example.com'
public_ip: false
default: True