Update ganeti-os-interface documentation

man/ganeti-os-interace.sgml lacked complete information for the NIC-related
environment variables. Added a reference to NIC_%N_LINK and NIC_%N_MODE and
clarified the reference to NIC_%N_BRIDGE.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>