[meta] DRY udp/tcp/graphite input plugins specs & spec helpers

The udp, tcp and graphite input plugins are defining pretty much the same spec helper. It should be DRY'ed up.

Also, @purbon suggested some spec refactors in logstash-plugins/logstash-input-graphite/pull/3 which we should followup on.

Compare the spec helpers in the udp, tcp, and graphite input plugin specs, then review the suggested refactors in logstash-plugins/logstash-input-graphite/pull/3. Consolidate the duplicated helper setup and apply the agreed spec refactors; done means the three plugin specs remain equivalent and their tests pass.

Written by the indexing model from the issue text.