osd: fix logging of host:disk:journal

Two problems:

1) if one of the elements of the 3-tuple are None, this
breaks (like, say, if you're allowing journal to default).  This is
#4498

2) multiple tuples just aren't handled correctly; need two separate
comprehensions

Fixes: #4498
Signed-off-by: Dan Mick <dan.mick@inktank.com>