Setup autoscaling for Gitter bastion

As a follow-up to #5492 and gitlab-com/gl-infra/production#559, we introduced a bastion host for access to the Gitter environment, instead of the legacy VPN. The new bastion is a single instance, with no redundancy for fault-tolerance or HA as a quick mitigation to gitlab-com/gl-infra/production#559, but now we need to shore up the infrastructure.

1. Setup a Network Load Balancer for SSH traffic to the bastion node
2. Setup an Autoscaling group for bastion nodes
3. Setup a Launch Configuration / Launch Template for bastion nodes
4. Provision a deterministic host key on bootstrap for all bastion nodes (allows clients to rely on strict host key checking)