Initial import of listrunner

This tool was used and worked on internally for quite a long time. We
decided to include it in Ganeti.

Known issues:
- Code doesn't match rest of Ganeti (e.g. using “print” all over the
  place, hardcoded calls to sys.exit deep in functions)
- Code duplication from Ganeti library (e.g. PingByTcp/netutils.TcpPing,
  GetHosts/utils.ReadFile)
- Using ssh-agent doesn't work with more than one worker (Paramiko keeps
  the socket open and the file descriptor is used from different
  workers)
  - No clear separation between parent and child process in code
- Uses getopt instead of optparse

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>