[WIP] : Refactoring to extract logic from the main func

[serbrech]

The goal of this PR is to clean up the main.go code to extract the logic and make it testable.

• [x] add KeyvaultFlexvolumeAdapter struct
• [x] move main() logic into a Run() function applied to the struct
• [x] add package github.com/pkg/errors
  • provides the commonly used errors.Wrapf()
• [x] wrap and return all errors from Run()
  • simplifies error handling
• [x] use glog.Fatalf instead of Exit(1)
  • Note: this changes the error exit code from 1 to 255 as per glog.Fatalf definition
  • Provides stacktrace on errors

TODO :

• [x] Apply functions to the adapter
• [x] Move KeyvaultFlexvolumeAdapter in its own file
• [ ] Add first tests

More :

• [ ] Abstract writeContent to enable testing