qa: use stdin-killer to timeout run_shell_payload

qa: use stdin-killer to timeout run_shell_payload

- simplify argument processing / forwarding
- use stdin-killer to kill all sub-processes of the shell
- do not needlessly use run_shell to execute the command as it adds a timeout
  to the stdout/stderr processing
- provide a stdin (PIPE) by default otherwise teuthology's code closes stdin
  and triggers stdin-killer to timeout the shell.
- use a 15 minute timeout by default

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>