Allow object store for Job Artifacts

Given that storage of artifacts on disk is not the most scalable
solution, object storage, e.g. S3 should be supported. The change
requires a new key in the `gitlab.yml`, `object_store` under artifacts.
If this is set to true, fog will be used for storage instead the local
FS.

If credentials are incorrect, we don't catch this yet.

app/uploaders/artifact_uploader.rb

 class ArtifactUploader < GitlabUploader
-  storage :file
-  attr_accessor :build, :field
+  storage Gitlab.config.artifacts.object_store ? :fog : :file
+  attr_accessor :job, :field
   def self.artifacts_path
     Gitlab.config.artifacts.path
@@ -15,16 +15,16 @@ class ArtifactUploader < GitlabUploader
     File.join(self.artifacts_path, 'tmp/cache/')
   end
-  def initialize(build, field)
-    @build, @field = build, field
+  def initialize(job, field)
+    @job, @field = job, field
   end
   def store_dir
-    File.join(self.class.artifacts_path, @build.artifacts_path)
+    File.join(self.class.artifacts_path, job.artifacts_path)
   end
   def cache_dir
-    File.join(self.class.artifacts_cache_path, @build.artifacts_path)
+    File.join(self.class.artifacts_cache_path, job.artifacts_path)
   end
   def filename