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
Merge "Added note for decommissioning block storage nodes"