Use /etc/wikimetrics config files for alembic upgrade

'alembic upgrade head' was using ./wikimedia/config/ config files
for database configuration instead of /etc/wikimetrics/ config files,
the ones setup by puppet.

./wikimetrics/run.py provides a way to override local config file paths
using environment variables. So the fix hard-sets the env variables
to point to /etc/wikimetrics, whenever the puppet config files exist.

I know the code doesn't look beautiful, suggestions are welcome!

Bug: 64294
Change-Id: I854c57d89c7b80db06ecc98c49a379b4ccc6a247