Save temporary files in STORAGE_DIR

This makes it trivial to save uploaded files to STORAGE_DIR, as the
temporary files Flask saves them in are already there.

tests/test_file_storage.py

@@ -0,0 +1,23 @@
+import os
+import tempfile
+
+from common_test_class import AbstractPillarTest
+
+
+class TempDirTest(AbstractPillarTest):
+    def test_tempfiles_location(self):
+        # After importing the application, tempfiles should be created in the STORAGE_DIR
+        storage = self.app.config['STORAGE_DIR']
+        self.assertEqual(os.environ['TMP'], storage)
+        self.assertNotIn('TEMP', os.environ)
+        self.assertNotIn('TMPDIR', os.environ)
+
+        handle, filename = tempfile.mkstemp()
+        os.close(handle)
+        dirname = os.path.dirname(filename)
+        self.assertEqual(dirname, storage)
+
+        tmpfile = tempfile.NamedTemporaryFile()
+        dirname = os.path.dirname(tmpfile.name)
+        self.assertEqual(dirname, storage)
+