[terraform] Make number of secondary nodes configured by the user

Summary:
This allows the user to configure the amount of secondary nodes they want to launch.

I think that we'll want to consider some more advanced auto-scaling logic in the future to automatically launch secondary nodes, but configuring this to be
user-defined for now

Depends on D12725

Test Plan: terraform apply. Setting variable to 2 caused 2 secondary nodes tasks to deploy

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12729