Stop hardcoding root user

Some parts of the code still use a hardcoded user name: root. This patch
replaces all with a constant specified at build time. The end goal is to
make it possible to run a Ganeti cluster without any special privileges
(of course this will prevent some functionality from working).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>