Add support for Paramiko exec_command's 'timeout' kwarg

[yvwulei]

How to use command_timeout in fabric2.4

Thanks first.

[bitprophet]

This has been an in-code TODO for a while:

fabric/fabric/runners.py

Lines 30 to 31 in e9939d6

	# TODO: pass in timeout= here when invoke grows timeout functionality
	# in Runner/Local.

The "real" ticket for this is at the command-runner (Invoke) layer: pyinvoke/invoke#539

The code Fabric's level of things needs to be tickling, in Paramiko, is: https://github.com/paramiko/paramiko/blob/cf7d49d66f3b1fbc8b0853518a54050182b3b5eb/paramiko/client.py#L463

[bitprophet]

See #539 / #645

[mcouthon]

After reading pyinvoke/invoke#539, I see that this was solved in invoke, but AFAICT, the code in fabric's master branch still does not pass the timeout param to command_exec.
Does this mean that this ability is only available when running fabric through the CLI?