Fixtures for different DBMS versions

[bohanjason]

This is not urgent. But I think we can have a better way to store the fixtures for different database versions. For example, Most of knobs and metrics in Postgres 9.2, 9.3, 9.4, 9.6 are same. We can only store these common knobs/metrics in a single Postgres fixture, and store the version-specific knobs/metrics separately. For now we have duplicated knobs/metrics in all Postgres versions.

ottertune/server/website/website/fixtures/