Added autoscan (prescan) at the beginning of the manual scan.

When manual scan of a single IP address is scheduled, there's a possibility that the given address doesn't have ``snmp_name`` and ``snmp_community`` yet. In such case, there's a need to get those values by performing autoscan (prescan) first - without them, instead of pairing the data from plugins with an existing device, scan may incorrectly report a new one.