Drop username from return values of identity_user_info

Users would have a non-null username only with Identity API v2 which
was available in Keystone of OpenStack Pike. Identity API v2 has been
removed since OpenStack Queens [1].

Function search_users() from OpenStack SDK still returns the username
attribute because of [2][3] but it will always be None since
Keystone's API does not return username since OpenStack Pike.

[1] https://docs.openstack.org/releasenotes/keystone/queens.html
[2] 975cabbdd8/openstack/cloud/_utils.py (L246)
[3] 76b081efe4

Change-Id: I2054dd55662698dabd0f2b3565c31dcd3bf24e5a
(cherry picked from commit 5fc8fca06b)

plugins/modules/identity_user_info.py

@@ -82,7 +82,7 @@ openstack_users:
             returned: success
             type: str
         name:
-            description: Name given to the user.
+            description: Username of the user.
             returned: success
             type: str
         enabled:
@@ -102,7 +102,7 @@ openstack_users:
             returned: success
             type: str
         username:
-            description: Username of the user
+            description: Username with Identity API v2 (OpenStack Pike or earlier) else Null
             returned: success
             type: str
 '''