Quick mod to stop an info leak.

bootlogs were persisting after experiment termination, only being replaced
when a particular node failed during TBSETUP (and hence sent back a boot
log to be stashed).  This was leaking info such as project and experiment
names, uids, groups.

For now the bootlog is being cleared as nodes come in to an experiment
via experiment swapin (inside nalloc).  running sched_reload or
sched_reserve will also call nalloc if the node is free, hence clearing
the bootlog as well.