[backup/terraform] use json logs in backup if COMM_SERVICES_USE_JSON_LOGS variable set to true

Summary: Configures backup to use JSON logs when COMM_SERVICES_USE_JSON_LOGS is true

Test Plan:
Successfully deployed custom backup image to staging with COMM_SERVICES_USE_JSON_LOGS set to true.
Confirmed JSON logs through AWS console

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13941