The compute API 2.20 microversion allows attaching and detaching
a volume to/from a server with status SHELVED or SHELVED_OFFLOADED.
For OSC this just means the user has to specify the appropriate
minimum microversion to make that work, so this change mentions
that in the "server add volume" and "server remove volume" command
description.
Related to nova blueprint volume-ops-when-shelved
Change-Id: I4824175e5d9e124e3bd9e9a8fd5a89277efc6cff
3df5f92b44