fixes #8739 - use global setting when hostgroup root_pass is blank

A blank (not nil) root_pass on hostgroups was preventing correct inheritance
from settings in both hosts and nested hostgroups.

root_pass was also being copied from parents/settings to hostgroups when they
were saved, when it should remain blank to continue inheriting.  Only hosts
are meant to copy the password on save.

Correct, but it wasn't tested anywhere.  The changes to root/grub password handling had no tests either.