Fix broken DescribeSnapshots handling.

aliyun.ecs.connection.EcsConnection().descibe_snapshot() was broken, it
now just calls describe_snapshots itself. Which itself was messed up
because it wasn't passing properly-formatted SnapshotIDs parameters.
Merge pull request #8 from quixey/OPS-1844-python-aliyun-snapshotting

Fix broken DescribeSnapshots handling.