add provider to iosxr shared module

This commit adds a new argument `provider` to the iosxr shared module that
allows common connection parameters to be passed as a dict object.  The
constraints on the args still applies.  This commit also updates the iosxr
doc fragment.

lib/ansible/utils/module_docs_fragments/iosxr.py

@@ -48,5 +48,12 @@ options:
         the SSH session
     required: false
     default: null
+  provider:
+    description:
+      - Convience method that allows all M(iosxr) arguments to be passed as
+        a dict object.  All constraints (required, choices, etc) must be
+        met either by individual arguments or values in this dict.
+    required: false
+    default: null
 
   """