[terraform] mariadb on aws

Summary:
Sets up a mariadb instance within a vpc.

Depends on D12297

Test Plan: terraform apply. Still testing uploading local keyserver data to the mariadb instance. Will not land until properly tested

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12298