Refactor the storage to support multiple type.

[hwchiu]

1. Support multiple type of storage
2. Rename from storageProvider to storage (contains files)
3. Implement the FakeStorage for testing.
4. The empty function of NFSStorage (Implement in another PR)

[codecov-io]

Codecov Report

Merging #60 into develop will decrease coverage by 0.21%.
The diff coverage is 87.87%.

@@             Coverage Diff             @@
##           develop      #60      +/-   ##
===========================================
- Coverage    80.84%   80.63%   -0.22%     
===========================================
  Files           23       27       +4     
  Lines          637      692      +55     
===========================================
+ Hits           515      558      +43     
- Misses          88       99      +11     
- Partials        34       35       +1

Impacted Files	Coverage Δ
src/storageprovider/nfs.go	0% <0%> (ø)
src/server/handler_volume.go	82.5% <100%> (ø)	⬆️
src/server/route.go	62.79% <100%> (ø)	⬆️
src/kubernetes/deployment.go	100% <100%> (ø)
src/storageprovider/storage.go	100% <100%> (ø)
src/storageprovider/fake.go	100% <100%> (ø)
src/server/handler_storage.go	83.67% <94.11%> (ø)
... and 2 more

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