ability to use discovery returned string as hostname

even though discovery can now distinguish hosts with multiple interfaces and there's uniquity option, hosts are still added with their ips as hostnames.

it would be nice to add an option to add a host using returned string as hostname - this would be especially useful for various snmp devices where some snmp oid often contains useful string.

if the returned string contains characters that are not allowed in a hostname, they are replaced with underscores