RAPI changes for instance moves

Two new resources are added:
- /2/instances/$name/prepare-export
- /2/instances/$name/export

The documentation for the existing resource for creating instances is updated
for remote imports. The RAPI client is extended for the new resources.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>