functions: FreeBSD version of add_user_to_group()
create-stack-user: Allow sudo to any group in addition to any user

This is necessary to execute nova-compute with libvirtd group using
sudo.
nova: Resolve workaround of running nova-compute as root with sudo

Previously, sudo was used to execute nova-compute as root. This allows
to run as any user that is member of group defined by $LIBVIRT_GROUP.