[terraform] add Tunnelbroker configuration variable to self-host

Summary:
This adds a way to specify the terraform url in a self-hosted keyserver by setting the tunnelbroker url as an env variable

Depends on D12724

Test Plan: Tested with a keyserver with prod and staging credentials with corresponding tunnelbroker url and successfully connected.

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12725