[services] Blob/Backup - Read env vars in runtime instead of compile-time flags

Summary:
Depends on D3691

As suggested in https://phabricator.ashoat.com/D3498#98870 we want to avoid using compile-time flags and replace them with reading environment variables in the runtime.

Test Plan:

cd services
yarn test-backup-service
yarn test-backup-service-dev-mode
yarn run-backup-service
yarn run-backup-service-dev-mode
yarn test-blob-service
yarn test-blob-service-dev-mode
yarn run-blob-service
yarn run-blob-service-dev-mode

They all should work as before.

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3872