knet: Change nodeids to 8 bit for new knet compatibility

I've also put an assert in totemknet_member_add() to check
for invalid nodeids. Later on we need to fix the rest of the
corosync code to only use 8bit nodeids (or force people to use
UDPU if they want large nodeids).

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>