Better use of shelljs

[belemaire]

This pull request improves the way our codebase makes use of shelljs library throughout ern modules.

Here is what this PR contains :

• Relocate shelljs inside ern-util so that only one instance of it gets used in the process.
  Instead of importing shelljs as before through :
  import shell from 'shelljs'
  the new way to retrieve the process wide instance is now
  import { shell } from 'ern-util'

• Configure instance of sheljs in ern-local-cli entry

  • fatal = true
    This will make shelljs throw an error if a command is failing (before we were using throwIfShellCommandFail util function. This is not needed anymore all code has been cleanup accordingly part of this PR).

  • verbose = logLevel === 'debug' || logLevel === 'trace'
    This will make shelljs log every command line it is executing, only when platform log level is set to debug or trace (before we were directly doing log.debug of the command line in some cases. This is not needed anymore all code has been cleanup accordingly part of this PR)

• Removes all dependencies on shelljs module from ern- modules, as the dependency is not directly needed by the modules (they now consume shell from ern-util)