Add stdout and stderr to exception when ready script fails

When a command run through ssh fails, nodepool raises an exception
saying that it failed, but doesn't include any other information.
This patch adds in the output from stderr and stdout if output=True,
as well as the exact command run to help debug problems.

Also, added output parameter to the fake ssh client for testing.

Change-Id: If55643aef91c90b7c27fe4f532d51d9ef72b1ab4
Merge "Add stdout and stderr to exception when ready script fails"