Allow overriding of directory using SECRETS_DIR env var

[mikesir87]

I've been doing some experimenting with changing mount points for secrets and also have another use case where, in another orchestration engine, I'm putting the secrets in other places. Would be nice to use the same library and not introduce code changes/branches, but simply swap secret locations using environment variables. This allow that. Oh... and Merry Christmas! 🎄😄