Update checks for validate_certs in openstack_cloud_from_module

Change I51105f11565c5ff33b04add36259c8703af11240 moved validate_certs default from None to False.
This causes checks to fail in openstack_cloud_from_module as validate_certs is never None anymore.

This patch changes reverts the default back to None, and reflects this in the documentation.

Task: 41776
Story: 2008600
Change-Id: Ic79510f863cf3a39c3f5c6d99f61d335f92f9388

plugins/doc_fragments/openstack.py

@@ -61,7 +61,6 @@ options:
       - Whether or not SSL API requests should be verified.
       - Before Ansible 2.3 this defaulted to C(yes).
     type: bool
-    default: False
     aliases: [ verify ]
   ca_cert:
     description:

plugins/module_utils/openstack.py

@@ -120,7 +120,7 @@ def openstack_full_argument_spec(**kwargs):
         auth=dict(default=None, type='dict', no_log=True),
         region_name=dict(default=None),
         availability_zone=dict(default=None),
-        validate_certs=dict(default=False, type='bool', aliases=['verify']),
+        validate_certs=dict(default=None, type='bool', aliases=['verify']),
         ca_cert=dict(default=None, aliases=['cacert']),
         client_cert=dict(default=None, aliases=['cert']),
         client_key=dict(default=None, no_log=True, aliases=['key']),