Make it more convenient to substitute configuration files

[ChristopherHogan]

The tests need generic configuration files so they'll work anywhere, but when doing specific testing (on Ares for example) we need to customize the conf files more heavily. This means changing the conf files in the repo, but then remembering to undo those changes when creating a PR. It would be nice if the test suite accepted an override config file so developers can keep local config files on each platform they use.

[ChristopherHogan]

An environment variable HERMES_CONFIG_OVERRIDE should work.