test(k8s): add tests for the k8s package

[arschles]

to improve coverage to 43.8%:

ENG000656:steward aaronschlesinger$ go test -cover ./k8s
ok      github.com/deis/steward/k8s 0.068s  coverage: 43.8% of statements

[codecov-io]

Current coverage is 51.53% (diff: 100%)

Merging #237 into master will increase coverage by 5.93%

@@             master       #237   diff @@
==========================================
  Files            65         65          
  Lines          1467       1467          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            669        756    +87   
+ Misses          716        622    -94   
- Partials         82         89     +7   

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