Add the ability to load a remote environment values file (#1296)

Enables the user to specify a remote path for an environment values file, e.g.,

```yaml
environments:
  cluster-azure-us-west:
    values:
      - git::https://git.company.org/helmfiles/global/azure.yaml?ref=master
      - git::https://git.company.org/helmfiles/global/us-west.yaml?ref=master
  cluster-gcp-europe-west:
    values:
      - git::https://git.company.org/helmfiles/global/gcp.yaml?ref=master
      - git::https://git.company.org/helmfiles/global/europe-west.yaml?ref=master

releases:
  - ...
```

This is particularly useful when you co-locate helmfiles within your project repo but want to reuse the definitions in a global repo.

pkg/state/create_test.go

@@ -1,6 +1,7 @@
 package state
 
 import (
+	"github.com/roboll/helmfile/pkg/remote"
 	"io/ioutil"
 	"path/filepath"
 	"reflect"
@@ -108,7 +109,9 @@ bar: {{ readFile "bar.txt" }}
 	})
 	testFs.Cwd = "/example/path/to"
 
-	state, err := NewCreator(logger, testFs.ReadFile, testFs.FileExists, testFs.Abs, testFs.Glob, nil, nil, "").ParseAndLoad(yamlContent, filepath.Dir(yamlFile), yamlFile, "production", true, nil)
+	r := remote.NewRemote(logger, testFs.Cwd, testFs.ReadFile, testFs.DirectoryExistsAt, testFs.FileExistsAt)
+	state, err := NewCreator(logger, testFs.ReadFile, testFs.FileExists, testFs.Abs, testFs.Glob, nil, nil, "", r).
+		ParseAndLoad(yamlContent, filepath.Dir(yamlFile), yamlFile, "production", true, nil)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}