teuthology/task/install: add stdin-killer helper

teuthology/task/install: add stdin-killer helper

This helper tool runs commands which may or may not take data on stdin.
Like "daemon-helper", if stdin signals EOF, stdin-killer will kill the
command but only as a last resort. It forwards EOF to the command by
closing the command's stdin (pipe) and then waiting a configurable
amount of time for the command to gracefully exit.

Additionally, if stdout or stderr are hung up -- i.e. the ssh parent
process has terminated -- then stdin-killer also detects this and
initiates the graceful shutdown of the command. This is something
daemon-helper does not do.

In general, this tool is a superior replacement of the daemon-helper
tool because you can write to the command's stdin normally.

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