Server: Accept database username and password as env vars (and clean them up)

[jasonish]

Test and if it fails, allow username and password of the datastore to be provided as command line environments. This would allow EveBox to work with secret stores like Hashicorp Vault.

This EveBox process should sanitize these environment vars after reading them.

Reference: https://github.com/jasonish/evebox/issues/201