Feature: Designate which 'outputs' to live test

## Background

The `live-test` CLI command supports sending to defined outputs for a rule being tested. By default, this command will *not* mock out any of the outputs and will try to send the test alert to each one (aws/slack/pagerduty/etc).

## Desired Change

Make it easier to send to only one type of service when invoking the `live-test` command. This would allow testing services like Slack without having to test AWS services. This would have to perform some selective mocking of outputs (see [here](https://github.com/airbnb/streamalert/blob/6114f665ae83d02d505c47cd19de12752cab3cfb/stream_alert_cli/test.py#L652) and [here](https://github.com/airbnb/streamalert/blob/6114f665ae83d02d505c47cd19de12752cab3cfb/stream_alert_cli/helpers.py#L373)).

Start with the live-test CLI command and read the referenced sections of stream_alert_cli/test.py and stream_alert_cli/helpers.py to understand current output handling and mocking. Define how a selected output type is supplied to live-test, then verify that only the requested service is exercised while other outputs are mocked.

Written by the indexing model from the issue text.