This adds some notes about two things to consider with a volume
retype operation on an in-use multiattach volume:
1. The volume API will reject a retype operation on an in-use volume
where the volume's multiattach value would change.
2. Retyping an in-use volume that has more than one active read/write
attachment is not recommended because on the nova side the swap
volume operation performs a blockRebase which could potentially
lead to data corruption.
In the future, we (nova or cinder or both) will likely need to make
some changes as to what the default attach_mode is for secondary
attachments to a multiattach volume, since today we default to read/write
if it's not specified by the caller when making the attachment.
Change-Id: Ib5a2830ea3cd37236947677225252ef42edaa347
2e1d31a44a