Switch to GNU timeout-based implementation of SSH timeouts

[clintoncwolfe]

Description

As the title says, this PR switches the SSH timeout feature to use the GNU timeout utility (when available) to implement command timeouts instead of using a client-side thread and closing the session, which abandons processes on the server (See #677).

This severely limits the applicability of the feature. First, the timeout utility must be installed. As it is part of gnu coreutils, it is common on Linux and other systems, but not universal, so we must do a check for it. Second, as it is a command wrapper, it does not function properly with any command string that includes pipes. Notably, all InSpec sudo commands include pipes. So, we check for pipes, and do not attempt to do a timeout on any command containing pipes.

This PR retains some functionality while making the feature safe to use. It is a compromise between letting the processes continue to run as on #677, and withdrawing the feature entirely from the SSH transport.

Related Issue

Fixes #677

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New content (non-breaking change)
• Breaking change (a content change which would break existing functionality or processes)

Checklist:

• I have read the CONTRIBUTING document.

[Vasu1105]

Thanks @clintoncwolfe this looks good to me. May be having test cases around it will be good, if possible.