Specify db config file to access prod environment

As ./scripts/populate_tags script does not run inside wikimetrics,
it used default local database config file, which has no creds for
production environment. This fix sets the env variable
WIKIMETRICS_DB_CONFIG to '/etc/wikimetrics/db_config.yaml'
so that configurables.py can retrieve the production config
from this puppet file.

Bug: 73519
Change-Id: Iba3cc7490487d66cd34bae299d6d146eec9b0980