parseNode: don't lookup values in drained nodes

Currently parseNode skips looking for values in offline nodes, but tries
to read them for drained ones. With this patch we treat offline and
drained nodes in the same way (which is compatible with the iallocator
protocol changes introduced in Ganeti 2.0.4, but also doesn't break
retrocompatibility). As a bonus we don't need to calculate the value of
the internal Node "offline" parameter, because we know that the node is
going to be online anyway, if we lookup values (and it was statically
set to offline, before, in the offline case).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>