Allow specifying a non-writable folder when not uploading a tarball but pointing to a existing config directory

I wonder if this is testable? The profile has tests for both an existing directory and a tarball.. perhaps the test could make the folder not writeable after putting the config in it. We also still need to check it exists even if we're not doing an upload.

+++ b/src/Form/SyncConfigureForm.php
@@ -71,7 +71,7 @@ class SyncConfigureForm extends FormBase {
+    if ($sync_directory != config_get_config_directory(CONFIG_SYNC_DIRECTORY) && $has_upload) {

This condition does not look quite right.

I think we should create if it is not there. If this fails we should error because this is a mistake.
Then if we have an upload we need to ensure it is writable.

Tried to write a test. But we can't just do it because chmodding to 0444 always results in 0644. Manually tested this and it is all good.

Drupal test run
---------------

Tests to be run:
  - Drupal\config_installer\Tests\ConfigInstallerEnSecondTest
  - Drupal\config_installer\Tests\ConfigInstallerFrDirectorySyncTest
  - Drupal\config_installer\Tests\ConfigInstallerFrTarballTest
  - Drupal\config_installer\Tests\ConfigInstallerNoDependenciesProfile
  - Drupal\config_installer\Tests\ConfigInstallerSyncTest
  - Drupal\config_installer\Tests\ConfigInstallerTarballTest
  - Drupal\config_installer\Tests\UninstalledProfileModulesTest

Test run started:
  Monday, February 27, 2017 - 18:29

Test summary
------------

Drupal\config_installer\Tests\ConfigInstallerEnSecondTest     36 passes
Drupal\config_installer\Tests\ConfigInstallerFrDirectorySync  32 passes
Drupal\config_installer\Tests\ConfigInstallerFrTarballTest    35 passes
Drupal\config_installer\Tests\ConfigInstallerNoDependenciesP  28 passes
Drupal\config_installer\Tests\ConfigInstallerSyncTest         33 passes
Drupal\config_installer\Tests\ConfigInstallerTarballTest      39 passes
Drupal\config_installer\Tests\UninstalledProfileModulesTest   45 passes

Test run duration: 1 min 43 sec

No test fails.