Move AWS configs from env vars to constance

[robhudson]

I saw that jezdez is wanting to move away from env vars, so I'm moving these to constance.

[codecov-io]

Codecov Report

Merging #611 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
+ Coverage   88.02%   88.03%   +<.01%     
==========================================
  Files          50       50              
  Lines        1670     1671       +1     
  Branches      158      158              
==========================================
+ Hits         1470     1471       +1     
  Misses        180      180              
  Partials       20       20

Impacted Files	Coverage Δ
atmo/clusters/provisioners.py	100% <ø> (ø)	⬆️
atmo/settings.py	91.46% <ø> (ø)	⬆️
atmo/jobs/provisioners.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2c9f5b1...db10e61. Read the comment docs.

[wlach]

The code changes look fine, I don't have a lot of context for the motivation behind the change but I trust your judgement :)

[robhudson]

The move to make these configurable is to allow the staging instance of ATMO to use the staging AWS buckets and profiles for EMR bootstrapping, so we can more easily test our bootstrapping scripts.

The move to constance instead of environment variables stems from this:
#225

Thanks for the review!