We don't have any api-ref for server-migrations. os-migrations
is frozen and server-migrations replaces it, but we don't
have any docs for it, so this is a start.
Partial-Bug: #1641713
Part of blueprint api-ref-in-rst-ocata
Change-Id: I3da6ff6a4a0abf2f9c8bae3fb28fe23fdc3b0959
