Extend module_defaults by adding default groups for cloud modules (#44127)

Extends `module_defaults` by adding a prefix to defaults `group/` which denotes a builtin list of modules. Initial groups are: `group/aws`, `group/azure`, and `group/gcp`

docs/docsite/rst/user_guide/module_defaults_config.rst

@@ -0,0 +1,24 @@
+.. _module_defaults_config:
+
+Module Defaults Configuration
+=============================
+
+Ansible 2.7 adds a preview-status feature to group together modules that share common sets of parameters. This makes
+it easier to author playbooks making heavy use of API-based modules such as cloud modules. By default Ansible ships
+with groups for AWS and GCP modules that share parameters.
+
+In a playbook, you can set module defaults for whole groups of modules, such as setting a common AWS region.
+
+.. code-block:: YAML
+
+    # example_play.yml
+    - hosts: localhost
+      module_defaults:
+        group/aws:
+          region: us-west-2
+      tasks:
+      - aws_s3_bucket_facts:
+      # now the region is shared between both facts modules
+      - ec2_ami_facts:
+          filters:
+            name: 'RHEL*7.5*'