validation: allowing for nested directories

```
=== RUN   TestValidateStorageShareDirectoryName
--- PASS: TestValidateStorageShareDirectoryName (0.00s)
    storage_test.go:49: [DEBUG] Test Input ""
    storage_test.go:49: [DEBUG] Test Input "abc123"
    storage_test.go:49: [DEBUG] Test Input "123abc"
    storage_test.go:49: [DEBUG] Test Input "123-abc"
    storage_test.go:49: [DEBUG] Test Input "-123-abc"
    storage_test.go:49: [DEBUG] Test Input "123-abc-"
    storage_test.go:49: [DEBUG] Test Input "123--abc"
    storage_test.go:49: [DEBUG] Test Input "hello/world"
    storage_test.go:49: [DEBUG] Test Input "hello/"
PASS
```

azurerm/helpers/validate/storage.go

@@ -11,7 +11,8 @@ func StorageShareDirectoryName(v interface{}, k string) (warnings []string, erro
 
 	// File share names can contain only lowercase letters, numbers, and hyphens,
 	// and must begin and end with a letter or a number.
-	if !regexp.MustCompile(`^[a-z0-9][a-z0-9-]+[a-z0-9]$`).MatchString(value) {
+	// however they can be nested (e.g. foo/bar)
+	if !regexp.MustCompile(`^[a-z0-9][a-z0-9-]+[a-z0-9]$`).MatchString(value) && !regexp.MustCompile(`^[a-z0-9]{1,}/[a-z0-9]{1,}$`).MatchString(value) {
 		errors = append(errors, fmt.Errorf("%s must contain only lowercase alphanumeric characters, numbers and hyphens. It must start and end with a letter and end only with a number or letter", k))
 	}
 

azurerm/helpers/validate/storage_test.go

@@ -35,6 +35,14 @@ func TestValidateStorageShareDirectoryName(t *testing.T) {
 			Input:    "123--abc",
 			Expected: false,
 		},
+		{
+			Input:    "hello/world",
+			Expected: true,
+		},
+		{
+			Input:    "hello/",
+			Expected: false,
+		},
 	}
 
 	for _, v := range testCases {