Add note about removing leading tabs in ceph.conf files

As reported in bug #1914814, common ways to generate ceph config files
result in files that have leading tabs. These tabs make Kolla Ansible's
ini parser unhappy, so add a note to remind users to remove them.

Closes-Bug: #1914814
Change-Id: I4b06eae75bf238f2f093bfb76ba37c7f75dfd616

doc/source/reference/storage/external-ceph-guide.rst

@@ -30,6 +30,14 @@ Configuring External Ceph
 
 Ceph integration is configured for different OpenStack services independently.
 
+.. note::
+
+    Commands like ``ceph config generate-minimal-conf`` generate configuration
+    files that have leading tabs. These tabs break Kolla Ansible's ini parser.
+    Be sure to remove the leading tabs from your ``ceph.conf`` files when
+    copying them in the following sections.
+
+
 Glance
 ------