Setup NAT Gateways for all Gitter AZs

As a follow-up to #5492 and gitlab-com/gl-infra/production#559, we introduced a NAT gateway for egress from internal hosts within the Gitter environment, instead of the legacy VPN which also acted as a NAT instance.

The new gateway is currently serving traffic for all subnets, but is susceptible to disruption if Amazon suffers an outage of the Availability Zone where the NAT Gateway is provisioned, and/or traffic is disrupted between AZs.

Now that the issue from gitlab-com/gl-infra/production#559 has been mitigated, we need to shore up the infrastructure, and provision additional NAT gateways in each availability zone, and associate the local private subnets in those AZs.