provisioners/chef: DRY and unify encrypted data bag secret handling

Pull up encrypted data bag secret management into the base classes, thus
also unifying the functionality.

Especially this complements the issues:
  * Upload the secret to provisioning path also with chef-solo [GH-1246]
  * Delete the secret also with chef-client [GH-2712]
  * Remove the secret before uploading also with chef-client [GH-1111]