[terraform] Use default vpc, subnets, and internet gateway for self-host

Summary:
This removes the need to specify availability zones by using the vpc, subnets, and internet gateway default to each region.

Depends on D12324

Test Plan: Deployed set up to region specified in config. Mariadb database was accessible.

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12339