Bug: logs key required for rules that use datatypes

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What

#285 introduced data normalization.

An intended side effect was the the logs key was not required if the datatypes key is provided.

Example:

@rule(datatypes=['command'])
def alert_suspicious_wget(rec):
    results = fetch_values_by_datatype(rec, 'command')
    for result in results:
        if fnmatch(result, "wget *"):
            return true
    return false

However, this doesn't match reality. If you don't specify logs, you get the following:

StreamAlert [ERROR]: Invalid rule [...] - rule must have 'logs' declared

Workaround

Define all relevant logs in the logs key for rules that use data normalization.

Expectations

• if the datatypes key exists, logs is not required, it's optional
• if the datatypes key does not exist, logs is required

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This is now fixed