Enforce UTF-8 encoding for file I/O

[danielmitterdorfer]

Currently we rely on the platform encoding when doing file I/O. Usually the platform encoding is set to UTF-8 so it does not make a difference in many cases. However, we should strictly enforce UTF-8 encoding on all file I/O to avoid failures due to the platform encoding.

We should specifically watch out for these places:

• Writing / reading Rally's config file
• Reading bulk data files
• Writing / reading Elasticsearch config files
• Reading index config files for the metrics store
• Writing / reading race.json files
• Writing race report files

To test it, we can set the platform encoding with export LC_ALL = C.