Allow policy_arns to be attached, needed for AWS managed policies

[msooszalando]

This change is needed to correctly support AWS managed policies, see https://docs.aws.amazon.com/IAM/latest/UserGuide/access_policies_managed-vs-inline.html?shortFooter=true#aws-managed-policies for such policies. They are very useful and lower the burden of maintenance of policy documents.

[coveralls]

Coverage decreased (-0.04%) to 23.671% when pulling 6d783c45db387a838d740f3c47ad17493df16c7e on msooszalando:master into e8d3a0307fd017cc6afcbce356db72037b1eaef2 on zalando-stups:master.

[codecov-io]

Codecov Report

Merging #78 into master will decrease coverage by 0.03%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   23.66%   23.63%   -0.04%     
==========================================
  Files          22       22              
  Lines        1952     1955       +3     
==========================================
  Hits          462      462              
- Misses       1490     1493       +3

Impacted Files	Coverage Δ
sevenseconds/config/iam.py	7.03% <0%> (-0.17%)	:arrow_down:

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[szuecs]

@msooszalando please add your signature in the comment of your PR git commit -s and rebase to one commit.

Why do we need to change sevenseconds and can't just change our awsconfig and rollout the change with sevenseconds?

@sarnowski is this the change you talked about with me some days ago?

[mikkeloscar]

Managed policies are now supported since #108 Closing this!

[msooszalando]

Woooow! Yaaaay! :heart_eyes: