Add module defaults group

Adds the module defaults group microsoft.ad.domain to allow setting
common connection parameters for module that communicate with the domain
controller.

changelogs/fragments/default_options.yml

@@ -0,0 +1,5 @@
+minor_changes:
+  - >-
+    Added ``group/microsoft.ad.domain`` module defaults group for the ``computer``, ``group``, ``object_info``,
+    ``object``, ``ou``, and ``user`` module. Users can use this defaults group to set common connection options
+    for these modules such as the ``domain_server``, ``domain_username``, and ``domain_password`` options.

meta/runtime.yml

@@ -1 +1,9 @@
-requires_ansible: '>=2.14'
+requires_ansible: '>=2.14'
+action_groups:
+  domain:
+    - computer
+    - group
+    - object_info
+    - object
+    - ou
+    - user

plugins/doc_fragments/ad_object.py

@@ -79,6 +79,8 @@ class ModuleDocFragment:
   domain_password:
     description:
     - The password for I(domain_username).
+    - This can be set under the R(play's module defaults,module_defaults_groups)
+      under the C(group/microsoft.ad.domain) key.
     type: str
   domain_server:
     description: