Added func ValidateZone

The zone parameter provided in a Storage Class may erroneously be an empty string or contain only spaces and tab characters. Such situation shall be detected and reported as an error.

That's why the func ValidateZone was added.

pkg/volume/util_test.go

@@ -557,3 +557,23 @@ func TestZonesToSet(t *testing.T) {
 		}
 	}
 }
+
+func TestValidateZone(t *testing.T) {
+	functionUnderTest := "ValidateZone"
+
+	// First part: want an error
+	errCases := []string{"", " 	 	 "}
+	for _, errCase := range errCases {
+		if got := ValidateZone(errCase); got == nil {
+			t.Errorf("%v(%v) returned (%v), want (%v)", functionUnderTest, errCase, got, "an error")
+		}
+	}
+
+	// Second part: want no error
+	succCases := []string{" us-east-1a	"}
+	for _, succCase := range succCases {
+		if got := ValidateZone(succCase); got != nil {
+			t.Errorf("%v(%v) returned (%v), want (%v)", functionUnderTest, succCase, got, nil)
+		}
+	}
+}