Deal with Host/32 only

This is reference to rapid7/metasploit-framework#2044 . report_host shouldn't try to deal with CIDR ranges, unless that CIDR range is but one host.

Note that this still doesn't fix the problem described in RM#7877, where the bad data is explicitly "10.1.1.1/28". Need to find out how that host got there, and regardless, the import shouldn't bail completely, it should just error on that host and move on.