Stacker

Minor update to remove dependencies on stacker\_blueprints for tests, since it
resulted in a circular dependency. This is just a fix to get tests running again,
and results in no change in functionality.

This is a big release which introduces the new concepts of Blueprint Variables
and Lookups. A lot of folks contributed to this release - in both code, and just
testing of the new features. Thanks to:

kylev, oliviervg1, datadotworld, acmcelwee, troyready, danielkza, and ttarhan

Special thanks to mhahn who did the bulk of the heavy lifting in this release, and

- Fix empire minion ECR access [GH-70]
- Fix SQS Queue Policy issue w/ multiple SQS queues [GH-71]
- Simple ECR repository blueprint [GH-72]

- Remove version and family checking from RDS [GH-67]

- Add low-level security group rule blueprint [GH-56]
- Relax troposphere dependency [GH-64]

- Add Elasticsearch Domain [GH-47]
- Fix namespace issue in firehose blueprint [GH-48]
- Setup flake8 in CI, cleanup bad pep8 blueprnts [GH-50]
- Update empire blueprints to empire 0.11.0 & fix various bugs [GH-51]