Fixes environment file using correct YAML format

An internal unexpected exception is raised while create a stack with an
environment file which only has string content. We should handle the case
and raise the correct error message such as "environment file is not Yaml
format".

Change-Id: I4e124db6a8dca9cf811f151ae95058cdaae16a01
Closes-Bug: #1273974

heatclient/common/environment_format.py

@@ -26,7 +26,7 @@ def parse(env_str):
     '''Takes a string and returns a dict containing the parsed structure.
 
     This includes determination of whether the string is using the
-    JSON or YAML format.
+    YAML format.
     '''
     try:
         env = yaml.load(env_str, Loader=yaml_loader)
@@ -35,6 +35,9 @@ def parse(env_str):
     else:
         if env is None:
             env = {}
+        elif not isinstance(env, dict):
+            raise ValueError('The environment is not a valid '
+                             'YAML mapping data type.')
 
     for param in env:
         if param not in SECTIONS:

heatclient/tests/test_environment_format.py

@@ -49,6 +49,18 @@ parameters: }
 '''
         self.assertRaises(ValueError, environment_format.parse, env)
 
+    def test_parse_string_environment(self):
+        env = 'just string'
+        expect = 'The environment is not a valid YAML mapping data type.'
+        msg = self.assertRaises(ValueError, environment_format.parse, env)
+        self.assertIn(expect, msg)
+
+    def test_parse_document(self):
+        env = '["foo" , "bar"]'
+        expect = 'The environment is not a valid YAML mapping data type.'
+        msg = self.assertRaises(ValueError, environment_format.parse, env)
+        self.assertIn(expect, msg)
+
 
 class YamlParseExceptions(testtools.TestCase):