Before this fix, customers can disable async migration only by passing
`--metadata async_migrate=False` in volumes metadata. This fix adds an
option named `vnx_async_migrate` which could help set the default value
of async migration for whole backend. If customers don't pass in the
`async_migrate` in metadata, the value of this option will be used.
Change-Id: I8e59039da78ecc9fb5415a10ff2a7c26ee689171
Closes-bug: #1796825