cardiff: Adding more context in subtitle (plots)

When plotting graphs, it's important to put context in it as much as
possible.

This patch add the following information in the graphs :
- Specific benchmark information (like block-size or number of tested
  cores)
- Step host
- Scheduling affinity
- Number of hypervisors
- Hardware information : amount of cpu & type, amount of ram
- OS information: Linux distribution, kernel version, os arch