Improve logging for workerpool tasks by providing __repr__

Before it would log something like “starting task
(<ganeti.http.client._HttpClientPendingRequest object at 0x2aaaad176790>,)”,
which isn't really useful for debugging. Now it'll log “[…]
<ganeti.http.client._HttpClientPendingRequest
req=<ganeti.http.client.HttpClientRequest 172.24.x.y:1811 PUT /node_info at
0x2aaaaab7ed10> at 0x2aaaaab823d0>”.

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