[publishers] Reduce redundancy of "output=" and "publishers=" on @rule

## Background

With the advent of #900, we now have redundant information being provided to `@rule` constructors:

```
@Rule(
logs=['cloudtrail:misc'],
outputs=['slack:infra', 'pagerduty:high'],
publishers={
'slack:infra': [publisher_1, publisher_2],
'pagerduty:high': [publisher_3, publisher4],
}
)
```

Notably, the values in `outputs=` are duplicated in the keys of `publishers=` (when the requested publishers are in dict format). This increases the chance of accidentally registering a publisher to a nonexistent output, or vice versa.

## Desired Change

Refer to this comment: https://github.com/airbnb/streamalert/pull/900#discussion_r259464165

Start with the discussion linked from the issue, then trace the @Rule constructor and how its outputs= and publishers= arguments are consumed. Done means removing the duplicated output names while preserving publisher registration and preventing publishers from targeting nonexistent outputs.

Written by the indexing model from the issue text.