[packaging] utilize a lambda layer for rules in the rules engine lambda

[ryandeivert]

Background

As we move toward packaging improvements, we should consider using an AWS Lambda Layer for management of rules deployed to the rules engine.

Desired Change

Add support for zipping / applying a new lambda layer for streamalert rules without having to deploy the rule rules engine code.

Things to consider:

Third party dependencies used by rules that are not part of the 'core' rules engine code. Maybe have a secondary layer for these?