osdmap: spread replicas across hosts with default crush map

This is more often the case than not, and we don't have a good way to
magically know what size of cluster the user will be creating.  Better to
err on the side of doing the right thing for more people.

Fixes: #3785
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>