Add subunit2sql repository type

[mtreinish]

Right now there is only 1 really usable repository type, the file backed one. This works well but has a number of limitations. A long term goal of the subunit2sql project was to provide a more rich repository that could be used for something like stestr/testrepository. Most of the pieces are there in subunit2sql to enable this, the only missing one is writing attachments out from the database, but this isn't necessarily a blocker for an initial implementation. Once the repo type is there we can work on making it the eventual default in subunit2sql.