fixes #2127: added support for root password hashing other than MD5

This is an alternative (to https://github.com/theforeman/foreman/pull/1736) implementation. Instead of having a global setting, a per-os attribute has been introduced. 
They don't, it's hardcoded at MD5 atm. #2127 deals with root passwords only, support for password hashing for grub should be done separately, as it would mean changes to templates, etc, etc