Added note for decommissioning block storage nodes
Added note for decommissioning block storage nodes

When decommissioning a block storage node, it is advisable to prevent
any volumes from being allocated there before migrating volumes out of
the node. Added a recommendation in volume migration instructions for
Red Hat Enterprise Linux/Fedora/CentOS and Debian/Ubuntu.

Change-Id: I8ea4ef7d856cecc136fb1207353c905f17e4b6ee
Partial-Bug: #1279179

Akasurde/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals