secretservice: use designated domains in tests (RFC2606)

Update domains used in tests to used domains that are designated for this
purpose as described in [RFC2606, section 3][1]

[1]: https://www.rfc-editor.org/rfc/rfc2606.html#section-3

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

secretservice/secretservice_test.go

@@ -13,7 +13,7 @@ func TestSecretServiceHelper(t *testing.T) {
 	t.Skip("test requires gnome-keyring but travis CI doesn't have it")
 
 	creds := &credentials.Credentials{
-		ServerURL: "https://foobar.docker.io:2376/v1",
+		ServerURL: "https://foobar.example.com:2376/v1",
 		Username:  "foobar",
 		Secret:    "foobarbaz",
 	}