dnsbl: moved rfc1918 checks from _base into dnsbl

Moving the checks into plugins/dnsbl properly excludes 127 from DNSBL queries while permitting the dnsbl_list_base tests to execute.
dns_list_base: moved init_redis into new method
dns_list_base: remove net_utils
Merge pull request #651 from msimerson/dnsbl

dnsbl: moved rfc1918 checks from _base into dnsbl