Fix paths to pid file, log file, configurations directory.

Addresses #117

Changes:
- Do not automatically set `MONGO_ORCHESTRATION_HOME` when unset.
- Use current working directory for pid and log files, or `MONGO_ORCHESTRATION_HOME` if set. Currently these may go into the site-packages directory, and `mongo-orchestration` may require root privileges to run.
- Look in parent directory of `mongo_orchestration` for "configurations" directory, or in `MONGO_ORCHESTRATION_HOME` if set. Currently we're looking in the `mongo_orchestration` package itself.

Basically, `MONGO_ORCHESTRATION_HOME` overrides the path where we look for or put things. Otherwise, we do what makes sense.