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/conf.py

@@ -210,6 +210,17 @@ register(
     category_slug='jobs',
 )
 
+register(
+    'AWX_TASK_ENV',
+    field_class=fields.DictField,
+    default={},
+    label=_('Extra Environment Variables'),
+    help_text=_('Additional environment variables set for playbook runs, inventory updates, project updates, and notification sending.'),
+    category=_('Jobs'),
+    category_slug='jobs',
+    placeholder={'HTTP_PROXY': 'myproxy.local:8080'},
+)
+
 register(
     'STDOUT_MAX_BYTES_DISPLAY',
     field_class=fields.IntegerField,