Documentation : wrong LVM filter

- [ ] This doc is inaccurate in this way: ______
- [ ] This is a doc addition request.
- [x] I have a fix to the document that I can paste below including example: input and output.

Input
-----

devices {
...
filter = [ "a/sdb/", "r/.*/"]

Output
------

devices {
...
filter = [ "a/sdb/", "r/.*/"]
global_filter = [ "a/sdb/", "r/.*/"]

----------

We also need to use the global_filter key to prevent lvmetad and other process to scan those devices.
If we just use the filter key, indeed it prevent the usage of VM volumes but the nodes are created and it ca lead to conflicts.

Source : https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/logical_volume_manager_administration/lvm_filters (note at the end of the page)

-----------------------------------
Release: 12.0.1.dev4 on 2018-03-04 10:48
SHA: 0983a422fa3e089347b581d47907c3369eb144cf
Source: https://git.openstack.org/cgit/openstack/cinder/tree/doc/source/install/cinder-storage-install-rdo.rst
URL: https://docs.openstack.org/cinder/queens/install/cinder-storage-install-rdo.html