Add admin users into 'root' and 'adm' groups

The root and adm system groups are often used to allow read-only permission
to sensitive log files, devices and configuration. Considering we allow
password-less access to sudo, there is no real reason to restrict our own admin
accounts from reading these files -- and giving access at a user-level prevents
unnecessary priv-escalation via sudo.

This is particularly important where directories are 0750 or similar -- it will allow
us to inspect the contents without resorting to 'sudo -i'.

They are always present, hence no need for 'require' - and subsequent update
to bootstrap-formula