[terraform] create comm database on aws rds startup

Summary:
Our scripts take care of creating the comm database. However, AWS RDS initializes empty, causing
ecs tasks running the keyserver image to fail. This uses a null_resource to setup the comm database before launching keyserver nodes

Depends on D12688

Test Plan: On terraform apply, I no longer need to mysql remotely into aws rds to create the comm database. The comm database is first created and then the ecs tasks are launched

Reviewers: bartek, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12689