List migrations through Admin API

The os-migrations extension exposes endpoint to fetch all migrations.
The migrations can filtered by host and status. If cells are
enabled migrations can be listed for all cells or can be filtered for a
particular cell.

The route for fetching migrations for
a region is - v2/{tenant_id}/os-migrations. Filters can be passed as
query parameters -
v2/{tenant_id}/os-migrations?host=host1&status=finished&cell_name=Child

DocImpact

Change-Id: Id70dbece344a722b2dc8c593dd340ef747eb43d3
Implements: blueprint list-resizes-through-admin-api
Merge "List migrations through Admin API"