The list of available ones is here:
http://docs.doctrine-project.org/projects/doctrine-dbal/en/latest/reference/configuration.html#driver

Also, we know the ones that Sugar supports, so we can easily do a mapping and have support for them all, since this is only to confirm connection. DB2 isn't yet fully supported by Doctrine, but people are already working on fixing bugs (https://github.com/doctrine/dbal/pull/447) on latest versions so I think we can bet on this.