Move api client out of cli.py

[tlkamp]

This PR moves api_client out of cli.py and instead creates an api_client in the main loop. Since cli.py no longer provides the api_client, the api_client is a now a function argument for functions that need a client. This makes mocking/testing significantly easier.

[codecov-io]

Codecov Report

Merging #38 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   96.45%   96.35%   -0.11%     
==========================================
  Files           7        7              
  Lines         141      137       -4     
==========================================
- Hits          136      132       -4     
  Misses          5        5

Impacted Files	Coverage Δ
ouroboros/cli.py	100% <ø> (ø)	⬆️
ouroboros/main.py	90.47% <100%> (+0.23%)	⬆️
ouroboros/image.py	100% <100%> (ø)	⬆️
ouroboros/container.py	97.36% <100%> (-0.07%)	⬇️

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[circa10a]

Heck of a PR, very clean!