Add tool to move instances between clusters

This is a first version of the instance move tool and it supports moving
1..N instances from one cluster to another. When moving a single instance,
the instance can be renamed, allowing for moves within the same cluster
(not really useful in practice, but good for testing). Documentation is
updated to describe this new tool and its usage.

The “move-instance” tool uses the workerpool to support parallel moves of
instances. Supporting them was simple as threads were required anyway due
to the synchronous RAPI client.

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