Create community rule for Trusted Advisor events

By default, StreamAlert monitors all CloudWatch events, but does not alert on all of them. The AWS Trusted Advisor service supposedly sends events to CloudWatch events by default. The TrustedAdvisor service checks for things like opening Security Groups publicly to port 3306 (mysql), making S3 buckets public, and a number of other checks. Some of these are free, and for those with Business support plans, there are alerts for things like Access Keys being leaked to Github.

We should have a Community rule to detect these, similar to what we have for GuardDuty: https://github.com/airbnb/streamalert/blob/master/rules/community/guardduty/guard_duty_all.py

Examples of these events are here: https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/EventTypes.html#trusted-advisor-event-types

Start by reading rules/community/guardduty/guard_duty_all.py and the AWS Trusted Advisor event examples linked in the issue. Use those event types to define the Community rule, then verify that it detects the relevant Trusted Advisor events without alerting on unrelated CloudWatch events.

Written by the indexing model from the issue text.