md_config_t: change from class to struct as the name indicates

Change md_config_t from class to struct to fix some issues
from clang. The name indicates it's a struct so change it to
struct and unify the usaged type.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #205 from dalgaaf/wip-da-md_config-class

md_config_t: change from class to struct as the name indicates

Reviewed-by: Greg Farnum <greg@inktank.com>