Simplify and speedup the catalog comparator.

- Removed rvm as it's unnecessary and slows us down
- Created separated envs for puppet 2 and 3 to remove lock contentions
- Use mysql as a backend for stored configs
- Threading enhancements: now the whole compilation is an execution unit.

Change-Id: I346ecde33d543fe1657a995f01e0f6aec5badff0
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>