support unix socket http endpoint for local only instances of kapacitor.

problem description:
currently run kapacitor on each instance to process alerts for each server reducing the need to aggregate metrics into influxdb first.

currently kapacitor seems to only allow setup of the http endpoint on a tcp socket. would love to be able to use a unix socket as an alternative.

No files, tests, or configuration keys are named. Start by locating Kapacitor's HTTP endpoint configuration and TCP listener entry point, then trace how a local Unix socket could be selected; done means local-only instances can serve the HTTP endpoint through a Unix socket without removing TCP support.

Written by the indexing model from the issue text.