Define {open,net}bsd_shadow vars in ChangePassword

Those variables are needed to make ChangePassword work

snf-image-helper/tasks/50ChangePassword.in

@@ -35,6 +35,8 @@ check_if_excluded
 
 linux_shadow="/etc/shadow"
 freebsd_shadow="/etc/master.passwd"
+openbsd_shadow="/etc/master.passwd"
+netbsd_shadow="/etc/master.passwd"
 
 linux_change_shadow_entry() {
     local line encrypted