fix AWS plugin credential precedence for environment variables (#52945)

* fix AWS plugin credential precedence for environment variables

* Allow aliases in direct plugins options

Consolidate precedence fix just in the doc fragment using aliases for mismatched options

* Access options with the option name rather than alias

* fix indentation

* update unit tests

* Improve readability

lib/ansible/plugins/doc_fragments/aws_region.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+
+# Copyright: (c) 2017,  Ansible Project
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+
+class ModuleDocFragment(object):
+
+    # Plugin option for AWS region
+    DOCUMENTATION = r'''
+options:
+  region:
+    description: The region for which to create the connection.
+    type: str
+    env:
+      - name: EC2_REGION
+      - name: AWS_REGION
+'''