Implement environment singleton to be accessed throughout the code

Load and parse environment file from working dir

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/helpers.py

@@ -1,9 +1,14 @@
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
+import functools
+import os
+
+from . import mock
 from compose.config.config import ConfigDetails
 from compose.config.config import ConfigFile
 from compose.config.config import load
+from compose.config.environment import Environment
 
 
 def build_config(contents, **kwargs):
@@ -14,3 +19,11 @@ def build_config_details(contents, working_dir='working_dir', filename='filename
     return ConfigDetails(
         working_dir,
         [ConfigFile(filename, contents)])
+
+
+def clear_environment(f):
+    @functools.wraps(f)
+    def wrapper(self, *args, **kwargs):
+        Environment.reset()
+        with mock.patch.dict(os.environ):
+            f(self, *args, **kwargs)