[services] Tunnelbroker - Changing the ConfigManager load function to include the sandbox condition

Summary: Changing the ConfigManager.load() function to include the sandbox config condition to avoid duplications and fix some of the tests.

Test Plan: Run build with tests: yarn run-tunnelbroker-service-in-sandbox or build inside the Docker container with the export COMM_TEST_SERVICES=1 command before the cmake command.

Reviewers: tomek, karol

Reviewed By: tomek, karol

Subscribers: ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4638