Add test for yaml enviroment

tests/util/test_yaml.py

@@ -1,6 +1,7 @@
 """Test Home Assistant yaml loader."""
 import io
 import unittest
+import os
 
 from homeassistant.util import yaml
 
@@ -32,3 +33,23 @@ class TestYaml(unittest.TestCase):
             pass
         else:
             assert 0
+
+    def test_enviroment_variable(self):
+        """Test config file with enviroment variable."""
+        os.environ["PASSWORD"] = "secret_password"
+        conf = "password: !env_var PASSWORD"
+        with io.StringIO(conf) as f:
+            doc = yaml.yaml.safe_load(f)
+        assert doc['password'] == "secret_password"
+        del os.environ["PASSWORD"]
+
+    def test_invalid_enviroment_variable(self):
+        """Test config file with no enviroment variable sat."""
+        conf = "password: !env_var PASSWORD"
+        try:
+            with io.StringIO(conf) as f:
+                yaml.yaml.safe_load(f)
+        except Exception:
+            pass
+        else:
+            assert 0