Support AWX_TASK_ENV injection in task and notification invocations.

This change _only_ injects `AWS_TASK_ENV` into `os.environ`; it's up to
underlying libraries to be good citizens and actually respect things
like `HTTPS_PROXY`.

see: #3508

awx/main/tests/unit/utils/test_common.py

@@ -2,7 +2,9 @@
 
 # Copyright (c) 2017 Ansible, Inc.
 # All Rights Reserved.
+import os
 import pytest
+from uuid import uuid4
 
 from awx.main.utils import common
 
@@ -15,3 +17,13 @@ from awx.main.utils import common
 ])
 def test_parse_yaml_or_json(input_, output):
     assert common.parse_yaml_or_json(input_) == output
+
+
+def test_set_environ():
+    key = str(uuid4())
+    old_environ = os.environ.copy()
+    with common.set_environ(**{key: 'bar'}):
+        assert os.environ[key] == 'bar'
+        assert set(os.environ.keys()) - set(old_environ.keys()) == set([key])
+    assert os.environ == old_environ
+    assert key not in os.environ